<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:e038d80f-b69f-470d-8399-2695eb35e24d,g:db19f5bb-a1af-44bc-aa7d-af2ba9906e27-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.bandwidth.com/role/Cover" xlink:type="simple" xlink:href="band-20251231.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitiesTable_beee9e58-b062-497e-a367-5ed8e10c2330" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntitiesTable_beee9e58-b062-497e-a367-5ed8e10c2330" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_c4c0224c-969e-4772-b009-0e933569b445" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntitiesTable_beee9e58-b062-497e-a367-5ed8e10c2330" xlink:to="loc_us-gaap_StatementClassOfStockAxis_c4c0224c-969e-4772-b009-0e933569b445" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c4c0224c-969e-4772-b009-0e933569b445_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c4c0224c-969e-4772-b009-0e933569b445" xlink:to="loc_us-gaap_ClassOfStockDomain_c4c0224c-969e-4772-b009-0e933569b445_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_50178119-5a0e-4cfc-ba75-89cf5de0484f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c4c0224c-969e-4772-b009-0e933569b445" xlink:to="loc_us-gaap_ClassOfStockDomain_50178119-5a0e-4cfc-ba75-89cf5de0484f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_1ff6e44d-5698-40a9-9e8b-cee9c9994157" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_50178119-5a0e-4cfc-ba75-89cf5de0484f" xlink:to="loc_us-gaap_CommonClassAMember_1ff6e44d-5698-40a9-9e8b-cee9c9994157" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_f03847ea-30da-4af9-bb77-0477fbc74499" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_50178119-5a0e-4cfc-ba75-89cf5de0484f" xlink:to="loc_us-gaap_CommonClassBMember_f03847ea-30da-4af9-bb77-0477fbc74499" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_503aca5e-5241-47b9-bc24-aaaf0dcb0ea3" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_DocumentType_503aca5e-5241-47b9-bc24-aaaf0dcb0ea3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentAnnualReport_250dd668-8deb-48fd-8594-b579d1956ae4" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentAnnualReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_DocumentAnnualReport_250dd668-8deb-48fd-8594-b579d1956ae4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_e0d8e421-defb-4871-9ef6-4e5e7cfc3861" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_DocumentPeriodEndDate_e0d8e421-defb-4871-9ef6-4e5e7cfc3861" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_15a17a72-d8e7-4412-ab8c-a9c614d9d8cf" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_CurrentFiscalYearEndDate_15a17a72-d8e7-4412-ab8c-a9c614d9d8cf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_83b62143-901e-4e72-9937-832fd0a2a192" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_DocumentTransitionReport_83b62143-901e-4e72-9937-832fd0a2a192" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_6e04ea78-db96-4e0f-b8fd-1c932cf97d77" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityFileNumber_6e04ea78-db96-4e0f-b8fd-1c932cf97d77" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_205fd31d-688b-4090-a17e-8b542b2e2e56" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityRegistrantName_205fd31d-688b-4090-a17e-8b542b2e2e56" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_a203ed51-4c3e-4fba-8463-13a09f1e9346" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityIncorporationStateCountryCode_a203ed51-4c3e-4fba-8463-13a09f1e9346" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_78463970-2e8c-4748-8980-c9117e748f0d" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityTaxIdentificationNumber_78463970-2e8c-4748-8980-c9117e748f0d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_bf962268-ed92-4188-aee2-8597833a023b" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityAddressAddressLine1_bf962268-ed92-4188-aee2-8597833a023b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_f800f03b-5560-415c-b571-98a4490fdcac" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityAddressCityOrTown_f800f03b-5560-415c-b571-98a4490fdcac" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_6da0c482-3843-45a7-9d3f-44e64cb5601b" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityAddressStateOrProvince_6da0c482-3843-45a7-9d3f-44e64cb5601b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_55a41ab3-9355-4010-9229-91e466324685" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityAddressPostalZipCode_55a41ab3-9355-4010-9229-91e466324685" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_feb3fad4-c2b7-43d5-ae4d-4766013def97" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_CityAreaCode_feb3fad4-c2b7-43d5-ae4d-4766013def97" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_ab5e8650-a3b9-470e-bd45-3ca207506ccf" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_LocalPhoneNumber_ab5e8650-a3b9-470e-bd45-3ca207506ccf" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_050b3be2-1fac-43e3-854f-48a61c03751c" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_Security12bTitle_050b3be2-1fac-43e3-854f-48a61c03751c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_4fecce06-9671-4784-af0a-0476645b75a2" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_TradingSymbol_4fecce06-9671-4784-af0a-0476645b75a2" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_da4b84ea-0f0b-417a-9638-12eadbd6553b" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_SecurityExchangeName_da4b84ea-0f0b-417a-9638-12eadbd6553b" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityWellKnownSeasonedIssuer_b4c6f6ba-706d-47d1-b957-e2ffa96e01c2" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityWellKnownSeasonedIssuer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityWellKnownSeasonedIssuer_b4c6f6ba-706d-47d1-b957-e2ffa96e01c2" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityVoluntaryFilers_103a9708-f487-49fe-9c3c-b0bfa1dc5eb3" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityVoluntaryFilers"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityVoluntaryFilers_103a9708-f487-49fe-9c3c-b0bfa1dc5eb3" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_a2896d0d-6e15-4e39-a07e-da4c5e8272ad" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityCurrentReportingStatus_a2896d0d-6e15-4e39-a07e-da4c5e8272ad" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_c68c5a11-6f95-4524-996e-d65ca9caf707" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityInteractiveDataCurrent_c68c5a11-6f95-4524-996e-d65ca9caf707" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_bea249f0-7d60-4fbe-b49c-df7250506fa4" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityFilerCategory_bea249f0-7d60-4fbe-b49c-df7250506fa4" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_d344e73c-02cc-45fa-95dc-976d401427d0" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntitySmallBusiness_d344e73c-02cc-45fa-95dc-976d401427d0" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_59e31296-8c4a-44c7-8607-53bde479001e" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityEmergingGrowthCompany_59e31296-8c4a-44c7-8607-53bde479001e" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_IcfrAuditorAttestationFlag_71477753-fafc-4f32-9bca-e92d42960377" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_IcfrAuditorAttestationFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_IcfrAuditorAttestationFlag_71477753-fafc-4f32-9bca-e92d42960377" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFinStmtErrorCorrectionFlag_a7c7364d-b20f-4d05-9dca-c8b7bfeee89d" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFinStmtErrorCorrectionFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_DocumentFinStmtErrorCorrectionFlag_a7c7364d-b20f-4d05-9dca-c8b7bfeee89d" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_26bbaf3c-0def-40c8-aef1-0a84e458ffb8" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityShellCompany_26bbaf3c-0def-40c8-aef1-0a84e458ffb8" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityPublicFloat_57583d32-bad9-4b15-80c7-e72e3170ffa1" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityPublicFloat"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityPublicFloat_57583d32-bad9-4b15-80c7-e72e3170ffa1" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_5912d0ee-5cda-4d8a-ab1e-3abaa64a525a" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_5912d0ee-5cda-4d8a-ab1e-3abaa64a525a" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentsIncorporatedByReferenceTextBlock_db39d1bb-ca83-4882-ad35-950777e766af" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentsIncorporatedByReferenceTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_DocumentsIncorporatedByReferenceTextBlock_db39d1bb-ca83-4882-ad35-950777e766af" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_9cfd63df-e288-4c88-bdcb-3079bf17b681" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_EntityCentralIndexKey_9cfd63df-e288-4c88-bdcb-3079bf17b681" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_01721c65-ef85-4fca-87cd-ff26f19c2a59" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_DocumentFiscalYearFocus_01721c65-ef85-4fca-87cd-ff26f19c2a59" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_55ee5ebe-dc39-4148-aefa-f0c923c44bcf" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_DocumentFiscalPeriodFocus_55ee5ebe-dc39-4148-aefa-f0c923c44bcf" xlink:type="arc" order="33"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_d64e86a9-fe74-408b-817b-022cdee97700" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_1aae00bc-b353-425b-87bf-dc82eb0a7e11" xlink:to="loc_dei_AmendmentFlag_d64e86a9-fe74-408b-817b-022cdee97700" xlink:type="arc" order="34"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/ConsolidatedBalanceSheets" xlink:type="simple" xlink:href="band-20251231.xsd#ConsolidatedBalanceSheets"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/ConsolidatedBalanceSheets" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_8f30020b-fab5-4cd2-8857-710110315d09" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_fcb938d9-b6dd-4d33-8270-0275e252f930" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_8f30020b-fab5-4cd2-8857-710110315d09" xlink:to="loc_us-gaap_StatementTable_fcb938d9-b6dd-4d33-8270-0275e252f930" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_7a3623d4-c34e-4b4d-b7b8-468a6e7ab18e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_fcb938d9-b6dd-4d33-8270-0275e252f930" xlink:to="loc_us-gaap_StatementClassOfStockAxis_7a3623d4-c34e-4b4d-b7b8-468a6e7ab18e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_7a3623d4-c34e-4b4d-b7b8-468a6e7ab18e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_7a3623d4-c34e-4b4d-b7b8-468a6e7ab18e" xlink:to="loc_us-gaap_ClassOfStockDomain_7a3623d4-c34e-4b4d-b7b8-468a6e7ab18e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c58f2df9-72d1-4fcb-9a63-bc63a24782de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_7a3623d4-c34e-4b4d-b7b8-468a6e7ab18e" xlink:to="loc_us-gaap_ClassOfStockDomain_c58f2df9-72d1-4fcb-9a63-bc63a24782de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_1212267e-20d1-4eb2-b75e-efa10b39c7d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c58f2df9-72d1-4fcb-9a63-bc63a24782de" xlink:to="loc_us-gaap_CommonClassAMember_1212267e-20d1-4eb2-b75e-efa10b39c7d0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_56f34c6c-d5f6-406f-86d3-622686727fd3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c58f2df9-72d1-4fcb-9a63-bc63a24782de" xlink:to="loc_us-gaap_CommonClassBMember_56f34c6c-d5f6-406f-86d3-622686727fd3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8f30020b-fab5-4cd2-8857-710110315d09" xlink:to="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_a69212ea-4f77-4f46-94fc-2286dda05f24" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:to="loc_us-gaap_AssetsCurrentAbstract_a69212ea-4f77-4f46-94fc-2286dda05f24" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_6a4abcf4-9f3c-4f37-b6ff-c6f21c3163f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_a69212ea-4f77-4f46-94fc-2286dda05f24" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_6a4abcf4-9f3c-4f37-b6ff-c6f21c3163f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesCurrent_1f3e1839-99d6-485a-b48e-36bfaa22abfd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_a69212ea-4f77-4f46-94fc-2286dda05f24" xlink:to="loc_us-gaap_MarketableSecuritiesCurrent_1f3e1839-99d6-485a-b48e-36bfaa22abfd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_ec924bc6-1ae6-44ea-af08-06b1b066abbd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_a69212ea-4f77-4f46-94fc-2286dda05f24" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_ec924bc6-1ae6-44ea-af08-06b1b066abbd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCostsCurrent_1c72f6ab-abcf-43a4-aafe-1e34a84c47ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCostsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_a69212ea-4f77-4f46-94fc-2286dda05f24" xlink:to="loc_us-gaap_DeferredCostsCurrent_1c72f6ab-abcf-43a4-aafe-1e34a84c47ce" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_491843c7-b522-48c3-a17d-5ee341c10708" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_a69212ea-4f77-4f46-94fc-2286dda05f24" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_491843c7-b522-48c3-a17d-5ee341c10708" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_dfa3aba5-0d2f-45ed-a155-8cd830013350" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_a69212ea-4f77-4f46-94fc-2286dda05f24" xlink:to="loc_us-gaap_AssetsCurrent_dfa3aba5-0d2f-45ed-a155-8cd830013350" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_7509a774-d7b2-4ab4-932b-6f755591e2e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_7509a774-d7b2-4ab4-932b-6f755591e2e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_866123c2-904f-4465-95d9-66602adc1f0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_866123c2-904f-4465-95d9-66602adc1f0d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_9bc7b304-6972-4132-a8e4-a933bbb8d5f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_9bc7b304-6972-4132-a8e4-a933bbb8d5f3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCosts_440b1e22-3628-4187-91f4-cd98cdb13568" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:to="loc_us-gaap_DeferredCosts_440b1e22-3628-4187-91f4-cd98cdb13568" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_e90f3f74-639b-4c9f-bb4e-1d64911afb21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_e90f3f74-639b-4c9f-bb4e-1d64911afb21" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_93734972-52af-4a03-a5ec-34ee2975575e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:to="loc_us-gaap_Goodwill_93734972-52af-4a03-a5ec-34ee2975575e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_eed60f26-6925-4b0c-824a-0a599a9ef88f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_057ce8a3-4094-4c1f-b8a9-b04a33358eab" xlink:to="loc_us-gaap_Assets_eed60f26-6925-4b0c-824a-0a599a9ef88f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8f30020b-fab5-4cd2-8857-710110315d09" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_9c05d5d1-9642-46e5-be95-5e262fa5d3a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_9c05d5d1-9642-46e5-be95-5e262fa5d3a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_f9e956ef-03be-42cd-b55c-a397a63f46a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_9c05d5d1-9642-46e5-be95-5e262fa5d3a1" xlink:to="loc_us-gaap_AccountsPayableCurrent_f9e956ef-03be-42cd-b55c-a397a63f46a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_01c673da-e00c-4ce0-aa6f-ffe855a80cd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_9c05d5d1-9642-46e5-be95-5e262fa5d3a1" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_01c673da-e00c-4ce0-aa6f-ffe855a80cd5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_14fa1606-6003-43a0-a672-a6664c3194dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_9c05d5d1-9642-46e5-be95-5e262fa5d3a1" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_14fa1606-6003-43a0-a672-a6664c3194dc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_35f0ce37-8c16-4928-a1a4-f6a03918de08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_9c05d5d1-9642-46e5-be95-5e262fa5d3a1" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_35f0ce37-8c16-4928-a1a4-f6a03918de08" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtCurrent_ed6fe480-8de2-42b7-9294-9a9c5213dd22" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_9c05d5d1-9642-46e5-be95-5e262fa5d3a1" xlink:to="loc_us-gaap_ConvertibleDebtCurrent_ed6fe480-8de2-42b7-9294-9a9c5213dd22" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_ca2beb9b-41f7-4697-b3dc-f6be5107ca77" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_9c05d5d1-9642-46e5-be95-5e262fa5d3a1" xlink:to="loc_us-gaap_LiabilitiesCurrent_ca2beb9b-41f7-4697-b3dc-f6be5107ca77" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_7f67511d-da8b-4221-907a-bc994fe659ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_7f67511d-da8b-4221-907a-bc994fe659ab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_62b2eaa0-6a12-48ab-a0b4-4648f68db64f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_62b2eaa0-6a12-48ab-a0b4-4648f68db64f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_145c5628-e1fe-40fb-a45a-f11be6a77316" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_145c5628-e1fe-40fb-a45a-f11be6a77316" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_016a9ca5-f838-4d48-9248-ce1897b0f5df" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_016a9ca5-f838-4d48-9248-ce1897b0f5df" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_836cb27c-d74e-4adc-876a-3a41d8a4ee64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_836cb27c-d74e-4adc-876a-3a41d8a4ee64" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_6f87a661-44d3-4747-9ba8-36e15e5c2e9c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_Liabilities_6f87a661-44d3-4747-9ba8-36e15e5c2e9c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_e6ee5701-15ce-4287-a13f-44ed3e9077c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_CommitmentsAndContingencies_e6ee5701-15ce-4287-a13f-44ed3e9077c5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_87e13cce-ebd5-456a-89b8-1ef22a831e4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_StockholdersEquityAbstract_87e13cce-ebd5-456a-89b8-1ef22a831e4d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_87d2b96d-be93-4866-b8c9-9356fcbc1c0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_87e13cce-ebd5-456a-89b8-1ef22a831e4d" xlink:to="loc_us-gaap_PreferredStockValue_87d2b96d-be93-4866-b8c9-9356fcbc1c0f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_d827faae-9e90-4983-a667-feca13fbc844" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_87e13cce-ebd5-456a-89b8-1ef22a831e4d" xlink:to="loc_us-gaap_CommonStockValue_d827faae-9e90-4983-a667-feca13fbc844" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_8afc48e3-0edc-4c6c-8c85-f42f5354e8c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_87e13cce-ebd5-456a-89b8-1ef22a831e4d" xlink:to="loc_us-gaap_AdditionalPaidInCapital_8afc48e3-0edc-4c6c-8c85-f42f5354e8c3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_d0341411-0a0c-482f-ac0a-04948d28a87f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_87e13cce-ebd5-456a-89b8-1ef22a831e4d" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_d0341411-0a0c-482f-ac0a-04948d28a87f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_346902a9-4141-4adb-aa09-18b93087b03a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_87e13cce-ebd5-456a-89b8-1ef22a831e4d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_346902a9-4141-4adb-aa09-18b93087b03a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_62500697-5774-4956-9464-297b158916c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_87e13cce-ebd5-456a-89b8-1ef22a831e4d" xlink:to="loc_us-gaap_StockholdersEquity_62500697-5774-4956-9464-297b158916c1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_cff9279e-e326-42b1-8efa-e9530f1da245" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_e3d20ac9-0d94-46f2-a477-66e424bf95fd" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_cff9279e-e326-42b1-8efa-e9530f1da245" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="simple" xlink:href="band-20251231.xsd#ConsolidatedBalanceSheetsParenthetical"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_4b244d1b-d447-4fe8-9d58-e927c80296a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_StatementTable_4b244d1b-d447-4fe8-9d58-e927c80296a6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_c6d352ac-f737-47f2-ab5c-afcbca91196f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_4b244d1b-d447-4fe8-9d58-e927c80296a6" xlink:to="loc_us-gaap_StatementClassOfStockAxis_c6d352ac-f737-47f2-ab5c-afcbca91196f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c6d352ac-f737-47f2-ab5c-afcbca91196f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c6d352ac-f737-47f2-ab5c-afcbca91196f" xlink:to="loc_us-gaap_ClassOfStockDomain_c6d352ac-f737-47f2-ab5c-afcbca91196f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_6a906cda-844f-449a-b7c4-4cf8aa27a08e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c6d352ac-f737-47f2-ab5c-afcbca91196f" xlink:to="loc_us-gaap_ClassOfStockDomain_6a906cda-844f-449a-b7c4-4cf8aa27a08e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_6037f4d8-b923-468c-b1e1-5677d7fb1741" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_6a906cda-844f-449a-b7c4-4cf8aa27a08e" xlink:to="loc_us-gaap_CommonClassAMember_6037f4d8-b923-468c-b1e1-5677d7fb1741" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_9b7b66b7-8a6a-47b4-a4f6-4f592f26d16e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_6a906cda-844f-449a-b7c4-4cf8aa27a08e" xlink:to="loc_us-gaap_CommonClassBMember_9b7b66b7-8a6a-47b4-a4f6-4f592f26d16e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_c2e67649-85d2-4bf2-91c2-d8c3c27c8934" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_c2e67649-85d2-4bf2-91c2-d8c3c27c8934" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_79892d8f-fc15-46b8-a59c-b20074b97eb3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_79892d8f-fc15-46b8-a59c-b20074b97eb3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_14d9475a-6b48-4f77-b110-e19bcd66c30c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_PreferredStockSharesIssued_14d9475a-6b48-4f77-b110-e19bcd66c30c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_a37f20fd-4ec7-4448-9d4d-a60a1125cce3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_a37f20fd-4ec7-4448-9d4d-a60a1125cce3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_5af3d2b8-641b-4336-898e-7b7b6c143d16" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_5af3d2b8-641b-4336-898e-7b7b6c143d16" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_c987bd16-4b1b-414b-974d-2b866da30f2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_c987bd16-4b1b-414b-974d-2b866da30f2b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_b648bd25-e545-40ba-80a1-cc43ed956b04" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_CommonStockSharesIssued_b648bd25-e545-40ba-80a1-cc43ed956b04" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_cdc5a7aa-3503-4ad3-86c4-fa3bda41761c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_637fc2e7-f6ad-499f-9c91-2df475ed452d" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_cdc5a7aa-3503-4ad3-86c4-fa3bda41761c" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/ConsolidatedStatementsofChangesinStockholdersEquity" xlink:type="simple" xlink:href="band-20251231.xsd#ConsolidatedStatementsofChangesinStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/ConsolidatedStatementsofChangesinStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_225e03ca-2a54-4eaa-9dd0-edc735caa7e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_041a933a-319b-4f62-9fbe-e1942d0db940" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_225e03ca-2a54-4eaa-9dd0-edc735caa7e0" xlink:to="loc_us-gaap_StatementTable_041a933a-319b-4f62-9fbe-e1942d0db940" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_b6a3ebcd-3dd9-46d4-b083-ac101be9a7aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_041a933a-319b-4f62-9fbe-e1942d0db940" xlink:to="loc_us-gaap_StatementClassOfStockAxis_b6a3ebcd-3dd9-46d4-b083-ac101be9a7aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b6a3ebcd-3dd9-46d4-b083-ac101be9a7aa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b6a3ebcd-3dd9-46d4-b083-ac101be9a7aa" xlink:to="loc_us-gaap_ClassOfStockDomain_b6a3ebcd-3dd9-46d4-b083-ac101be9a7aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_807e9641-cd39-4550-8b7b-bca9b868239d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b6a3ebcd-3dd9-46d4-b083-ac101be9a7aa" xlink:to="loc_us-gaap_ClassOfStockDomain_807e9641-cd39-4550-8b7b-bca9b868239d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_785b55d6-2488-4c8c-9c75-5b5a75e35f64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_807e9641-cd39-4550-8b7b-bca9b868239d" xlink:to="loc_us-gaap_CommonClassAMember_785b55d6-2488-4c8c-9c75-5b5a75e35f64" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_1517b346-f3d4-4866-b9aa-642e1e3a1d88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_807e9641-cd39-4550-8b7b-bca9b868239d" xlink:to="loc_us-gaap_CommonClassBMember_1517b346-f3d4-4866-b9aa-642e1e3a1d88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_5e8b33e4-58d9-4ed2-b241-88ce25ce606d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_041a933a-319b-4f62-9fbe-e1942d0db940" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_5e8b33e4-58d9-4ed2-b241-88ce25ce606d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_5e8b33e4-58d9-4ed2-b241-88ce25ce606d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_5e8b33e4-58d9-4ed2-b241-88ce25ce606d" xlink:to="loc_us-gaap_EquityComponentDomain_5e8b33e4-58d9-4ed2-b241-88ce25ce606d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_5c1f1caa-66b7-4756-8f33-fda353b02678" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_5e8b33e4-58d9-4ed2-b241-88ce25ce606d" xlink:to="loc_us-gaap_EquityComponentDomain_5c1f1caa-66b7-4756-8f33-fda353b02678" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_d735edc6-aaa4-46b1-b024-db018bf85da6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5c1f1caa-66b7-4756-8f33-fda353b02678" xlink:to="loc_us-gaap_CommonStockMember_d735edc6-aaa4-46b1-b024-db018bf85da6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_6a2eaf11-d5e5-427d-a9f9-93779e97c9d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5c1f1caa-66b7-4756-8f33-fda353b02678" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_6a2eaf11-d5e5-427d-a9f9-93779e97c9d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_e4e683d5-1da9-4cc3-b17d-47dcf68b32bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5c1f1caa-66b7-4756-8f33-fda353b02678" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_e4e683d5-1da9-4cc3-b17d-47dcf68b32bb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_40fb6a16-7654-478a-81bb-a6f655e062d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5c1f1caa-66b7-4756-8f33-fda353b02678" xlink:to="loc_us-gaap_RetainedEarningsMember_40fb6a16-7654-478a-81bb-a6f655e062d9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_225e03ca-2a54-4eaa-9dd0-edc735caa7e0" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_1b01c33d-d9cf-423a-a18c-cc5b7c4a20ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_1b01c33d-d9cf-423a-a18c-cc5b7c4a20ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_47707080-0f67-4455-9736-fb762259f3ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_StockholdersEquity_47707080-0f67-4455-9736-fb762259f3ad" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_86a9082f-b8ff-40e6-b099-70c402bfdfbb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_86a9082f-b8ff-40e6-b099-70c402bfdfbb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_8e300b32-3ada-4461-bbe9-ed0832ec6e51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_8e300b32-3ada-4461-bbe9-ed0832ec6e51" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_cbb8a3d2-b819-4e3b-bc6f-42a470862c68" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_cbb8a3d2-b819-4e3b-bc6f-42a470862c68" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures_72491eb6-3e30-449f-98e8-b2de03870be9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures_72491eb6-3e30-449f-98e8-b2de03870be9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_992c5c98-807b-4139-8481-bd2d381e1db5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_992c5c98-807b-4139-8481-bd2d381e1db5" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_a40eb190-fa01-46fb-b986-dbc9f30621c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_a40eb190-fa01-46fb-b986-dbc9f30621c4" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_81e9801a-f24c-4870-89e4-6e2b097b3164" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_81e9801a-f24c-4870-89e4-6e2b097b3164" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_ddcda9a4-830e-4e7e-baf1-42af598ce777" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_ddcda9a4-830e-4e7e-baf1-42af598ce777" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesUnrealizedGainLoss_6375933a-5b18-405b-8772-21f2480d088c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesUnrealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_MarketableSecuritiesUnrealizedGainLoss_6375933a-5b18-405b-8772-21f2480d088c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_ce0d6f48-7557-4217-ad76-7fb525bcb185" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_ce0d6f48-7557-4217-ad76-7fb525bcb185" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent_1fb840e8-bab6-4f1d-88da-e7620651586b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent_1fb840e8-bab6-4f1d-88da-e7620651586b" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_6bc0faf5-a367-40dc-9258-915983e9fd3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_6bc0faf5-a367-40dc-9258-915983e9fd3b" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_82f58ad3-838e-4ecc-9384-49c31a4c28dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_c19cfab1-05b1-49c8-a924-f3a09679050f" xlink:to="loc_us-gaap_NetIncomeLoss_82f58ad3-838e-4ecc-9384-49c31a4c28dc" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_e2f5ea8c-27c0-4c5b-8a4b-4c417d1c6859" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_690d4dae-c8f7-4285-ad11-dc64a41da74e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesScheduleofRevenueByCategoryDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DescriptionofBusinessandSummaryofSignificantAccountingPoliciesScheduleofRevenueByCategoryDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesScheduleofRevenueByCategoryDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_85bdf33c-6b37-46ec-b87b-9b2cd2fe4d62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_b5a9b7f6-27ed-4d46-a7b4-9d7521bfba93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_85bdf33c-6b37-46ec-b87b-9b2cd2fe4d62" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_b5a9b7f6-27ed-4d46-a7b4-9d7521bfba93" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_fd9b6a22-1b06-44ae-9f2f-075fb31484f2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_b5a9b7f6-27ed-4d46-a7b4-9d7521bfba93" xlink:to="loc_srt_ProductOrServiceAxis_fd9b6a22-1b06-44ae-9f2f-075fb31484f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_fd9b6a22-1b06-44ae-9f2f-075fb31484f2_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_fd9b6a22-1b06-44ae-9f2f-075fb31484f2" xlink:to="loc_srt_ProductsAndServicesDomain_fd9b6a22-1b06-44ae-9f2f-075fb31484f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_dc917204-b273-4bcf-842c-b396120e537d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_fd9b6a22-1b06-44ae-9f2f-075fb31484f2" xlink:to="loc_srt_ProductsAndServicesDomain_dc917204-b273-4bcf-842c-b396120e537d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CloudCommunicationsMember_c9242e7a-c5d3-4a7a-8e8c-74d6ba208d7b" xlink:href="band-20251231.xsd#band_CloudCommunicationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_dc917204-b273-4bcf-842c-b396120e537d" xlink:to="loc_band_CloudCommunicationsMember_c9242e7a-c5d3-4a7a-8e8c-74d6ba208d7b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_MessagingSurchargesMember_50449fe7-f7f6-40c2-89e0-28ad11e9f4a0" xlink:href="band-20251231.xsd#band_MessagingSurchargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_dc917204-b273-4bcf-842c-b396120e537d" xlink:to="loc_band_MessagingSurchargesMember_50449fe7-f7f6-40c2-89e0-28ad11e9f4a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_14940469-2fe8-4002-b7f3-cc26208cfab1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_85bdf33c-6b37-46ec-b87b-9b2cd2fe4d62" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_14940469-2fe8-4002-b7f3-cc26208cfab1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DescriptionofBusinessandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_1d64bdce-fb1e-4f19-aeab-4b8719b00f1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_ConcentrationRiskTable_1d64bdce-fb1e-4f19-aeab-4b8719b00f1c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_b3f146e2-acb7-4a86-813c-c3b3bdda7515" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_1d64bdce-fb1e-4f19-aeab-4b8719b00f1c" xlink:to="loc_srt_ProductOrServiceAxis_b3f146e2-acb7-4a86-813c-c3b3bdda7515" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_b3f146e2-acb7-4a86-813c-c3b3bdda7515_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_b3f146e2-acb7-4a86-813c-c3b3bdda7515" xlink:to="loc_srt_ProductsAndServicesDomain_b3f146e2-acb7-4a86-813c-c3b3bdda7515_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_3cc9eadd-df9d-4018-b346-4f487611516c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_b3f146e2-acb7-4a86-813c-c3b3bdda7515" xlink:to="loc_srt_ProductsAndServicesDomain_3cc9eadd-df9d-4018-b346-4f487611516c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CPaaSUsageBasedFeesMember_c0d08355-03eb-476c-a040-5d5d58bc80bb" xlink:href="band-20251231.xsd#band_CPaaSUsageBasedFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_3cc9eadd-df9d-4018-b346-4f487611516c" xlink:to="loc_band_CPaaSUsageBasedFeesMember_c0d08355-03eb-476c-a040-5d5d58bc80bb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CPaaSServiceFeesMember_89b440dd-fb4f-4d1d-aa6a-9d4ff019b227" xlink:href="band-20251231.xsd#band_CPaaSServiceFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_3cc9eadd-df9d-4018-b346-4f487611516c" xlink:to="loc_band_CPaaSServiceFeesMember_89b440dd-fb4f-4d1d-aa6a-9d4ff019b227" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_4b6a0097-9ebf-4d16-afd0-ff07adc6f340" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_1d64bdce-fb1e-4f19-aeab-4b8719b00f1c" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_4b6a0097-9ebf-4d16-afd0-ff07adc6f340" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_6c58c7b5-9f13-41fb-be89-06e90fcc2acc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_6c58c7b5-9f13-41fb-be89-06e90fcc2acc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_6cb8773b-f31d-4afb-acbe-c08a93126f53" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_6cb8773b-f31d-4afb-acbe-c08a93126f53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_b026acea-e5a1-462d-815a-23531385911c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_b026acea-e5a1-462d-815a-23531385911c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_ba2990ff-b020-4bf7-af1a-fd41d63f285b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_ba2990ff-b020-4bf7-af1a-fd41d63f285b" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails_1" xlink:type="simple" xlink:href="band-20251231.xsd#DescriptionofBusinessandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails_1"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_1d64bdce-fb1e-4f19-aeab-4b8719b00f1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_ConcentrationRiskTable_1d64bdce-fb1e-4f19-aeab-4b8719b00f1c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_b3f146e2-acb7-4a86-813c-c3b3bdda7515" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_1d64bdce-fb1e-4f19-aeab-4b8719b00f1c" xlink:to="loc_srt_ProductOrServiceAxis_b3f146e2-acb7-4a86-813c-c3b3bdda7515" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_b3f146e2-acb7-4a86-813c-c3b3bdda7515_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_b3f146e2-acb7-4a86-813c-c3b3bdda7515" xlink:to="loc_srt_ProductsAndServicesDomain_b3f146e2-acb7-4a86-813c-c3b3bdda7515_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_3cc9eadd-df9d-4018-b346-4f487611516c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_b3f146e2-acb7-4a86-813c-c3b3bdda7515" xlink:to="loc_srt_ProductsAndServicesDomain_3cc9eadd-df9d-4018-b346-4f487611516c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CPaaSUsageBasedFeesMember_c0d08355-03eb-476c-a040-5d5d58bc80bb" xlink:href="band-20251231.xsd#band_CPaaSUsageBasedFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_3cc9eadd-df9d-4018-b346-4f487611516c" xlink:to="loc_band_CPaaSUsageBasedFeesMember_c0d08355-03eb-476c-a040-5d5d58bc80bb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CPaaSServiceFeesMember_89b440dd-fb4f-4d1d-aa6a-9d4ff019b227" xlink:href="band-20251231.xsd#band_CPaaSServiceFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_3cc9eadd-df9d-4018-b346-4f487611516c" xlink:to="loc_band_CPaaSServiceFeesMember_89b440dd-fb4f-4d1d-aa6a-9d4ff019b227" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_6c58c7b5-9f13-41fb-be89-06e90fcc2acc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_6c58c7b5-9f13-41fb-be89-06e90fcc2acc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_6cb8773b-f31d-4afb-acbe-c08a93126f53" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_6cb8773b-f31d-4afb-acbe-c08a93126f53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_b026acea-e5a1-462d-815a-23531385911c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_b026acea-e5a1-462d-815a-23531385911c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_ba2990ff-b020-4bf7-af1a-fd41d63f285b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_8c1e2634-02a2-4b65-a253-519889756a6b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_ba2990ff-b020-4bf7-af1a-fd41d63f285b" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesDeferredCostsandInternalUseSoftwareDevelopmentCostsDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DescriptionofBusinessandSummaryofSignificantAccountingPoliciesDeferredCostsandInternalUseSoftwareDevelopmentCostsDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesDeferredCostsandInternalUseSoftwareDevelopmentCostsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostLineItems_e51b0b34-89a2-4a42-96e0-449b219e922f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostTable_fc56b99d-5446-4a33-9c7d-4bb644be1899" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_e51b0b34-89a2-4a42-96e0-449b219e922f" xlink:to="loc_us-gaap_CapitalizedContractCostTable_fc56b99d-5446-4a33-9c7d-4bb644be1899" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_432d258d-1531-4e94-8008-40ee58e6ed7f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CapitalizedContractCostTable_fc56b99d-5446-4a33-9c7d-4bb644be1899" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_432d258d-1531-4e94-8008-40ee58e6ed7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_432d258d-1531-4e94-8008-40ee58e6ed7f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_432d258d-1531-4e94-8008-40ee58e6ed7f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_432d258d-1531-4e94-8008-40ee58e6ed7f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a9161b60-19f6-4a16-9d8a-64b547fe2412" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_432d258d-1531-4e94-8008-40ee58e6ed7f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a9161b60-19f6-4a16-9d8a-64b547fe2412" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_70d60043-b864-42bc-8ab6-78cbdb078a5f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a9161b60-19f6-4a16-9d8a-64b547fe2412" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_70d60043-b864-42bc-8ab6-78cbdb078a5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_8aab5e0e-178c-4055-8b2b-07fb093fde9a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_e51b0b34-89a2-4a42-96e0-449b219e922f" xlink:to="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_8aab5e0e-178c-4055-8b2b-07fb093fde9a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesScheduleofFiniteLivedIntangibleAssetsDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DescriptionofBusinessandSummaryofSignificantAccountingPoliciesScheduleofFiniteLivedIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesScheduleofFiniteLivedIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_2db8ad3f-006e-4363-8bf6-36dd7681589d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_cb897fe6-a3ba-4945-a910-e72bb0aeeb1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_2db8ad3f-006e-4363-8bf6-36dd7681589d" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_cb897fe6-a3ba-4945-a910-e72bb0aeeb1c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_8e3b8103-5d1f-46ae-8e14-137ee263c0b9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_cb897fe6-a3ba-4945-a910-e72bb0aeeb1c" xlink:to="loc_srt_RangeAxis_8e3b8103-5d1f-46ae-8e14-137ee263c0b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_8e3b8103-5d1f-46ae-8e14-137ee263c0b9_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_8e3b8103-5d1f-46ae-8e14-137ee263c0b9" xlink:to="loc_srt_RangeMember_8e3b8103-5d1f-46ae-8e14-137ee263c0b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_460eeb4c-8d64-4970-b692-f6e74ac9d173" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_8e3b8103-5d1f-46ae-8e14-137ee263c0b9" xlink:to="loc_srt_RangeMember_460eeb4c-8d64-4970-b692-f6e74ac9d173" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_d8ffcf64-faf9-47fb-854e-b6a04c6c3180" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_460eeb4c-8d64-4970-b692-f6e74ac9d173" xlink:to="loc_srt_MinimumMember_d8ffcf64-faf9-47fb-854e-b6a04c6c3180" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_369da88e-ee41-4cb0-938d-d9d14030b899" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_460eeb4c-8d64-4970-b692-f6e74ac9d173" xlink:to="loc_srt_MaximumMember_369da88e-ee41-4cb0-938d-d9d14030b899" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_97bac455-6efa-4998-8a25-7621b798a19c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_cb897fe6-a3ba-4945-a910-e72bb0aeeb1c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_97bac455-6efa-4998-8a25-7621b798a19c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_97bac455-6efa-4998-8a25-7621b798a19c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_97bac455-6efa-4998-8a25-7621b798a19c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_97bac455-6efa-4998-8a25-7621b798a19c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_31a5ff99-117b-443f-9787-332e28a3b7cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_97bac455-6efa-4998-8a25-7621b798a19c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_31a5ff99-117b-443f-9787-332e28a3b7cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_d65db539-84b1-4d43-9f3c-59611a95fc1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_31a5ff99-117b-443f-9787-332e28a3b7cc" xlink:to="loc_us-gaap_CustomerRelationshipsMember_d65db539-84b1-4d43-9f3c-59611a95fc1a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_5755b6d5-6238-4855-b8ba-89fe8900caab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_31a5ff99-117b-443f-9787-332e28a3b7cc" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_5755b6d5-6238-4855-b8ba-89fe8900caab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_c166d183-9fb3-4e85-ab4c-8dad90f62c62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_31a5ff99-117b-443f-9787-332e28a3b7cc" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_c166d183-9fb3-4e85-ab4c-8dad90f62c62" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_1e4ec6f9-853e-4f99-a868-1f6d1ee84a60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_2db8ad3f-006e-4363-8bf6-36dd7681589d" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_1e4ec6f9-853e-4f99-a868-1f6d1ee84a60" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesStockBasedCompensationDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DescriptionofBusinessandSummaryofSignificantAccountingPoliciesStockBasedCompensationDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DescriptionofBusinessandSummaryofSignificantAccountingPoliciesStockBasedCompensationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_dc18a7aa-b0f9-4293-94db-e7819b799345" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e6fa2f35-11fe-43b3-998c-9d6a021e94bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_dc18a7aa-b0f9-4293-94db-e7819b799345" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e6fa2f35-11fe-43b3-998c-9d6a021e94bf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_16bc4468-54d5-4fd3-9420-ca6fb2d51f0c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e6fa2f35-11fe-43b3-998c-9d6a021e94bf" xlink:to="loc_srt_RangeAxis_16bc4468-54d5-4fd3-9420-ca6fb2d51f0c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_16bc4468-54d5-4fd3-9420-ca6fb2d51f0c_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_16bc4468-54d5-4fd3-9420-ca6fb2d51f0c" xlink:to="loc_srt_RangeMember_16bc4468-54d5-4fd3-9420-ca6fb2d51f0c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_082cf8e4-ae8d-4210-9167-63bb51425e72" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_16bc4468-54d5-4fd3-9420-ca6fb2d51f0c" xlink:to="loc_srt_RangeMember_082cf8e4-ae8d-4210-9167-63bb51425e72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_ff5f94c6-81c1-49d5-9d84-6ded6a9e03d3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_082cf8e4-ae8d-4210-9167-63bb51425e72" xlink:to="loc_srt_MinimumMember_ff5f94c6-81c1-49d5-9d84-6ded6a9e03d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_dd44059e-5a4c-42e2-b2b7-3d7bb3f1402a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_082cf8e4-ae8d-4210-9167-63bb51425e72" xlink:to="loc_srt_MaximumMember_dd44059e-5a4c-42e2-b2b7-3d7bb3f1402a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_119aff13-82a9-4fba-8d3d-0d18bcd8b2d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e6fa2f35-11fe-43b3-998c-9d6a021e94bf" xlink:to="loc_us-gaap_AwardTypeAxis_119aff13-82a9-4fba-8d3d-0d18bcd8b2d2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_119aff13-82a9-4fba-8d3d-0d18bcd8b2d2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_119aff13-82a9-4fba-8d3d-0d18bcd8b2d2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_119aff13-82a9-4fba-8d3d-0d18bcd8b2d2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d129c85d-01d1-4581-a546-c079fc8d9991" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_119aff13-82a9-4fba-8d3d-0d18bcd8b2d2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d129c85d-01d1-4581-a546-c079fc8d9991" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_StockOptionsAndRestrictedStockUnitsEmployeesMember_2441a5a3-1704-4e08-9caa-61d3b19ab3cc" xlink:href="band-20251231.xsd#band_StockOptionsAndRestrictedStockUnitsEmployeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d129c85d-01d1-4581-a546-c079fc8d9991" xlink:to="loc_band_StockOptionsAndRestrictedStockUnitsEmployeesMember_2441a5a3-1704-4e08-9caa-61d3b19ab3cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_RestrictedStockUnitsNonEmployeeMember_f454c27c-491e-4637-9951-d5a5c3068688" xlink:href="band-20251231.xsd#band_RestrictedStockUnitsNonEmployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d129c85d-01d1-4581-a546-c079fc8d9991" xlink:to="loc_band_RestrictedStockUnitsNonEmployeeMember_f454c27c-491e-4637-9951-d5a5c3068688" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_2be10b87-0282-4bc0-b888-ce11b344eaf6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_dc18a7aa-b0f9-4293-94db-e7819b799345" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_2be10b87-0282-4bc0-b888-ce11b344eaf6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/FairValueMeasurementsScheduleofFinancialAssetsMeasuredatFairValueDetails" xlink:type="simple" xlink:href="band-20251231.xsd#FairValueMeasurementsScheduleofFinancialAssetsMeasuredatFairValueDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FairValueMeasurementsScheduleofFinancialAssetsMeasuredatFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_ebec54cd-92ac-4d8f-86db-81989bbf00a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_ebec54cd-92ac-4d8f-86db-81989bbf00a0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_280b0513-1abd-40b2-ae4c-c2f27326d7c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_ebec54cd-92ac-4d8f-86db-81989bbf00a0" xlink:to="loc_us-gaap_FinancialInstrumentAxis_280b0513-1abd-40b2-ae4c-c2f27326d7c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_280b0513-1abd-40b2-ae4c-c2f27326d7c6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_280b0513-1abd-40b2-ae4c-c2f27326d7c6" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_280b0513-1abd-40b2-ae4c-c2f27326d7c6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_0c35ceff-36e3-46cb-8021-ba2cc8c1c76d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_280b0513-1abd-40b2-ae4c-c2f27326d7c6" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_0c35ceff-36e3-46cb-8021-ba2cc8c1c76d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankTimeDepositsMember_e415e304-bfe3-4ca5-a752-a288d1b4b2f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankTimeDepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_0c35ceff-36e3-46cb-8021-ba2cc8c1c76d" xlink:to="loc_us-gaap_BankTimeDepositsMember_e415e304-bfe3-4ca5-a752-a288d1b4b2f3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019c70e9-b2e6-736c-863e-bd8d5db8e721" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_0c35ceff-36e3-46cb-8021-ba2cc8c1c76d" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019c70e9-b2e6-736c-863e-bd8d5db8e721" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_217a28cc-7d27-4514-b005-761e9ebef78b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_ebec54cd-92ac-4d8f-86db-81989bbf00a0" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_217a28cc-7d27-4514-b005-761e9ebef78b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_217a28cc-7d27-4514-b005-761e9ebef78b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_217a28cc-7d27-4514-b005-761e9ebef78b" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_217a28cc-7d27-4514-b005-761e9ebef78b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_511fe0b7-bd28-4323-b994-8092301d5cef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_217a28cc-7d27-4514-b005-761e9ebef78b" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_511fe0b7-bd28-4323-b994-8092301d5cef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_b32fdba4-20f7-47b4-a816-749ff9eb6276" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_511fe0b7-bd28-4323-b994-8092301d5cef" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_b32fdba4-20f7-47b4-a816-749ff9eb6276" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_85fa0894-32a1-4a54-96a9-a2021b32cc9a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_511fe0b7-bd28-4323-b994-8092301d5cef" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_85fa0894-32a1-4a54-96a9-a2021b32cc9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_e02d5710-4940-44dc-ae10-d79b1741a730" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_511fe0b7-bd28-4323-b994-8092301d5cef" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_e02d5710-4940-44dc-ae10-d79b1741a730" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019c4318-dd63-711e-bc4b-843e0e88e1eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_ebec54cd-92ac-4d8f-86db-81989bbf00a0" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019c4318-dd63-711e-bc4b-843e0e88e1eb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c4318-dd63-711e-bc4b-843e0e88e1eb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019c4318-dd63-711e-bc4b-843e0e88e1eb" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c4318-dd63-711e-bc4b-843e0e88e1eb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c4318-dd63-77ce-9224-0e06853a936d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019c4318-dd63-711e-bc4b-843e0e88e1eb" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c4318-dd63-77ce-9224-0e06853a936d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019c4319-4a9b-71e3-9dc3-fb6d219981b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c4318-dd63-77ce-9224-0e06853a936d" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019c4319-4a9b-71e3-9dc3-fb6d219981b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019c4319-4a9b-7f29-bc2a-4746d244794f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c4318-dd63-77ce-9224-0e06853a936d" xlink:to="loc_us-gaap_CommercialPaperMember_019c4319-4a9b-7f29-bc2a-4746d244794f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsMember_019c432c-ba78-7ccb-b166-2bb8abc546b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c4318-dd63-77ce-9224-0e06853a936d" xlink:to="loc_us-gaap_CashEquivalentsMember_019c432c-ba78-7ccb-b166-2bb8abc546b5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019c4a3d-020f-7ad6-a939-d41824c10868" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_ebec54cd-92ac-4d8f-86db-81989bbf00a0" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019c4a3d-020f-7ad6-a939-d41824c10868" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c4a3d-020f-7ad6-a939-d41824c10868_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019c4a3d-020f-7ad6-a939-d41824c10868" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c4a3d-020f-7ad6-a939-d41824c10868_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019c4a3d-807c-79d2-a321-b6d4bdb5ec66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019c4a3d-020f-7ad6-a939-d41824c10868" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019c4a3d-807c-79d2-a321-b6d4bdb5ec66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c4a3d-020f-7b2e-aec8-4521c2f07fcc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019c4a3d-807c-79d2-a321-b6d4bdb5ec66" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c4a3d-020f-7b2e-aec8-4521c2f07fcc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019c4a3d-4e0f-79b9-aaa0-3a32a0cf8794" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c4a3d-020f-7b2e-aec8-4521c2f07fcc" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019c4a3d-4e0f-79b9-aaa0-3a32a0cf8794" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_e4014902-86fd-4cef-a5fa-1aeb84163d07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_e4014902-86fd-4cef-a5fa-1aeb84163d07" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_9aeb0653-6d08-4d6a-875a-d7583391f852" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_9aeb0653-6d08-4d6a-875a-d7583391f852" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_a1ba540b-b062-4df3-93a2-f86cc63553d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_a1ba540b-b062-4df3-93a2-f86cc63553d7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_04579037-2069-4880-9bb0-9f5652aab9a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_04579037-2069-4880-9bb0-9f5652aab9a6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_7698eff6-17b8-485b-a040-7ece8584153c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_7698eff6-17b8-485b-a040-7ece8584153c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_631f5441-229f-44ee-8b19-5db96ecf9bfd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_631f5441-229f-44ee-8b19-5db96ecf9bfd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_e154df4e-310a-477a-933f-ec665bb4bfbc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_2000984d-befb-468d-94c2-ec0d83e748a8" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_e154df4e-310a-477a-933f-ec665bb4bfbc" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/FairValueMeasurementsAdditionalInformationDetails" xlink:type="simple" xlink:href="band-20251231.xsd#FairValueMeasurementsAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FairValueMeasurementsAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_b84fdef2-6551-47ca-bccb-5b24fd55d75f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_da5ade14-e1ff-4596-bd20-51567e34a066" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b84fdef2-6551-47ca-bccb-5b24fd55d75f" xlink:to="loc_us-gaap_DebtInstrumentTable_da5ade14-e1ff-4596-bd20-51567e34a066" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_a176d6e8-045d-4db9-a21e-8217e53c54f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_da5ade14-e1ff-4596-bd20-51567e34a066" xlink:to="loc_us-gaap_DebtInstrumentAxis_a176d6e8-045d-4db9-a21e-8217e53c54f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_a176d6e8-045d-4db9-a21e-8217e53c54f5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_a176d6e8-045d-4db9-a21e-8217e53c54f5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_a176d6e8-045d-4db9-a21e-8217e53c54f5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_5326956b-920c-43a3-a89d-b133fa0e2553" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_a176d6e8-045d-4db9-a21e-8217e53c54f5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_5326956b-920c-43a3-a89d-b133fa0e2553" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_c3fdc866-0068-4dd8-b20c-041e54621662" xlink:href="band-20251231.xsd#band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_5326956b-920c-43a3-a89d-b133fa0e2553" xlink:to="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_c3fdc866-0068-4dd8-b20c-041e54621662" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_fc0d7bdc-ece3-42ba-a4d0-72ed450e5427" xlink:href="band-20251231.xsd#band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_5326956b-920c-43a3-a89d-b133fa0e2553" xlink:to="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_fc0d7bdc-ece3-42ba-a4d0-72ed450e5427" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_b0e25052-567f-45bc-9c95-bcbc9e36d168" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_da5ade14-e1ff-4596-bd20-51567e34a066" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_b0e25052-567f-45bc-9c95-bcbc9e36d168" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_b0e25052-567f-45bc-9c95-bcbc9e36d168_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_b0e25052-567f-45bc-9c95-bcbc9e36d168" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_b0e25052-567f-45bc-9c95-bcbc9e36d168_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_fc03c67f-6614-4354-8420-d5e57c1c00d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_b0e25052-567f-45bc-9c95-bcbc9e36d168" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_fc03c67f-6614-4354-8420-d5e57c1c00d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_ac9ff60f-b717-4bb1-89a9-302dab6e50a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_fc03c67f-6614-4354-8420-d5e57c1c00d7" xlink:to="loc_us-gaap_ConvertibleDebtMember_ac9ff60f-b717-4bb1-89a9-302dab6e50a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_65d1d384-dc66-4fa7-8e2a-50915d92fd10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_da5ade14-e1ff-4596-bd20-51567e34a066" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_65d1d384-dc66-4fa7-8e2a-50915d92fd10" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_65d1d384-dc66-4fa7-8e2a-50915d92fd10_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_65d1d384-dc66-4fa7-8e2a-50915d92fd10" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_65d1d384-dc66-4fa7-8e2a-50915d92fd10_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_a2ea7c29-33f5-4cea-ac47-d082b7e4f3fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_65d1d384-dc66-4fa7-8e2a-50915d92fd10" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_a2ea7c29-33f5-4cea-ac47-d082b7e4f3fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_45b57680-34b5-404a-b1bc-dda2c7df0e07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_a2ea7c29-33f5-4cea-ac47-d082b7e4f3fb" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_45b57680-34b5-404a-b1bc-dda2c7df0e07" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities_053a9cf6-a020-4541-aa60-a01f94764116" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b84fdef2-6551-47ca-bccb-5b24fd55d75f" xlink:to="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities_053a9cf6-a020-4541-aa60-a01f94764116" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt_d37d7da1-8a2d-4e28-b1f5-5c2217b6e102" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b84fdef2-6551-47ca-bccb-5b24fd55d75f" xlink:to="loc_us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt_d37d7da1-8a2d-4e28-b1f5-5c2217b6e102" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterest_a61edca8-67c4-4214-bed3-7f30a3961f65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b84fdef2-6551-47ca-bccb-5b24fd55d75f" xlink:to="loc_us-gaap_InvestmentIncomeInterest_a61edca8-67c4-4214-bed3-7f30a3961f65" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_0d284a58-8b05-416b-8bd8-1841b9c77c0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b84fdef2-6551-47ca-bccb-5b24fd55d75f" xlink:to="loc_us-gaap_LongTermDebtFairValue_0d284a58-8b05-416b-8bd8-1841b9c77c0f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_6e956e91-bd9e-486a-a156-8fbf6d9adbd1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b84fdef2-6551-47ca-bccb-5b24fd55d75f" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_6e956e91-bd9e-486a-a156-8fbf6d9adbd1" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/LeasesAdditionalInformationDetails" xlink:type="simple" xlink:href="band-20251231.xsd#LeasesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/LeasesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_8d3bf881-688b-4a5c-ae66-38999be62628" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_8d3bf881-688b-4a5c-ae66-38999be62628" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_093e2175-f781-4872-8bdd-13343b76fec3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_8d3bf881-688b-4a5c-ae66-38999be62628" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_093e2175-f781-4872-8bdd-13343b76fec3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_093e2175-f781-4872-8bdd-13343b76fec3_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_093e2175-f781-4872-8bdd-13343b76fec3" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_093e2175-f781-4872-8bdd-13343b76fec3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_6f2e4928-a3bf-430b-9910-e32493b1f202" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_093e2175-f781-4872-8bdd-13343b76fec3" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_6f2e4928-a3bf-430b-9910-e32493b1f202" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CorporateHeadquartersMember_e8f68355-d941-47c2-8e16-b415fe36c93d" xlink:href="band-20251231.xsd#band_CorporateHeadquartersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_6f2e4928-a3bf-430b-9910-e32493b1f202" xlink:to="loc_band_CorporateHeadquartersMember_e8f68355-d941-47c2-8e16-b415fe36c93d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LessorOperatingLeaseRenewalTerm_ec84745b-d0bd-4c7b-8301-8cf0808ff6ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LessorOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_LessorOperatingLeaseRenewalTerm_ec84745b-d0bd-4c7b-8301-8cf0808ff6ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermLeaseCost_4a0141f9-d265-4e74-9d39-9b014e5ebecb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_ShortTermLeaseCost_4a0141f9-d265-4e74-9d39-9b014e5ebecb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_OperatingLeaseRightOfUseAssetAccumulatedAmortization_d0bbea2d-04c5-4a77-830e-342eb680bfef" xlink:href="band-20251231.xsd#band_OperatingLeaseRightOfUseAssetAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_band_OperatingLeaseRightOfUseAssetAccumulatedAmortization_d0bbea2d-04c5-4a77-830e-342eb680bfef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_78c7ba0a-b257-48b2-92c6-5d8f498be0f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_78c7ba0a-b257-48b2-92c6-5d8f498be0f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_LesseeOperatingLeaseNumberOfOptionToExtend_3eb89dd9-6a8a-445b-9485-d65eb6c47e80" xlink:href="band-20251231.xsd#band_LesseeOperatingLeaseNumberOfOptionToExtend"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_band_LesseeOperatingLeaseNumberOfOptionToExtend_3eb89dd9-6a8a-445b-9485-d65eb6c47e80" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_3a4201b1-ff71-4b91-ab7c-c97627c20fc2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_3a4201b1-ff71-4b91-ab7c-c97627c20fc2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_MarketRentalRate_019c31ab-e606-7976-bb6f-a2d454bfb0d4" xlink:href="band-20251231.xsd#band_MarketRentalRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_band_MarketRentalRate_019c31ab-e606-7976-bb6f-a2d454bfb0d4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_e0f4cf1e-c2c8-43db-beea-d1ee0fe5dab8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_e0f4cf1e-c2c8-43db-beea-d1ee0fe5dab8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_6ceac490-40f8-44d3-9e7f-91463189fa99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_4f142d3f-81ca-4197-be96-23b0a24905cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_OperatingLeaseLiability_4f142d3f-81ca-4197-be96-23b0a24905cc" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_LeaseIncentive_5d1aff3f-4e09-4b64-a491-657012a716a6" xlink:href="band-20251231.xsd#band_LeaseIncentive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_band_LeaseIncentive_5d1aff3f-4e09-4b64-a491-657012a716a6" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecurityDeposit_184dc47d-dc46-4fd7-b185-e251a316af87" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecurityDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_SecurityDeposit_184dc47d-dc46-4fd7-b185-e251a316af87" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EscrowDeposit_cc3104d4-07b8-42b4-8af1-24b4f6661d2f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EscrowDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_EscrowDeposit_cc3104d4-07b8-42b4-8af1-24b4f6661d2f" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromLeasePayments_dd10b56f-59de-45ab-9393-86a60778932d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_ProceedsFromLeasePayments_dd10b56f-59de-45ab-9393-86a60778932d" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LessorOperatingLeasePaymentsToBeReceived_384f0965-731b-42bc-bd2f-eb8402ccbf54" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LessorOperatingLeasePaymentsToBeReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_5a6c26a5-a310-4424-92c7-caf704bd3168" xlink:to="loc_us-gaap_LessorOperatingLeasePaymentsToBeReceived_384f0965-731b-42bc-bd2f-eb8402ccbf54" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/PropertyPlantandEquipmentScheduleofPropertyPlantandEquipmentNetDetails" xlink:type="simple" xlink:href="band-20251231.xsd#PropertyPlantandEquipmentScheduleofPropertyPlantandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/PropertyPlantandEquipmentScheduleofPropertyPlantandEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c562b49b-ce1e-43c0-be4d-72d478aa7647" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_cc61fad7-4908-40c6-acf5-037dacde60df" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c562b49b-ce1e-43c0-be4d-72d478aa7647" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_cc61fad7-4908-40c6-acf5-037dacde60df" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1ac52b0a-e54f-4363-a8da-b4a4df87c37c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_cc61fad7-4908-40c6-acf5-037dacde60df" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1ac52b0a-e54f-4363-a8da-b4a4df87c37c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1ac52b0a-e54f-4363-a8da-b4a4df87c37c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1ac52b0a-e54f-4363-a8da-b4a4df87c37c" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1ac52b0a-e54f-4363-a8da-b4a4df87c37c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1ac52b0a-e54f-4363-a8da-b4a4df87c37c" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_eec7ed6d-1653-451f-adf7-9dae5f60ca80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_eec7ed6d-1653-451f-adf7-9dae5f60ca80" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ComputerAndOfficeEquipmentMember_06dc8892-c2c3-4005-a206-a084c2d748be" xlink:href="band-20251231.xsd#band_ComputerAndOfficeEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_band_ComputerAndOfficeEquipmentMember_06dc8892-c2c3-4005-a206-a084c2d748be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_TelecommunicationsEquipmentMember_bf1b032a-c243-4e6e-b226-8a144ddd041b" xlink:href="band-20251231.xsd#band_TelecommunicationsEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_band_TelecommunicationsEquipmentMember_bf1b032a-c243-4e6e-b226-8a144ddd041b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_af6f7522-4d73-4669-b523-1ba043a6fa14" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_af6f7522-4d73-4669-b523-1ba043a6fa14" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_93574143-8836-4741-a236-072abba7d155" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_93574143-8836-4741-a236-072abba7d155" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_70238881-27ab-4759-bbd8-a84b1b90542e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_70238881-27ab-4759-bbd8-a84b1b90542e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AutomobilesMember_e43700b6-47e3-4d60-a9f4-710c14ad83de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AutomobilesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_us-gaap_AutomobilesMember_e43700b6-47e3-4d60-a9f4-710c14ad83de" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandMember_096682a8-4fa6-41f1-b173-aef7d29c2f6f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_us-gaap_LandMember_096682a8-4fa6-41f1-b173-aef7d29c2f6f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandImprovementsMember_dcfd73af-c7c6-4ef4-ba64-d1c0105b8a7d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LandImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_92fe7c17-9d85-4872-aabe-0d63c4c4c9f0" xlink:to="loc_us-gaap_LandImprovementsMember_dcfd73af-c7c6-4ef4-ba64-d1c0105b8a7d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_ba80e84d-260e-4116-b40e-92fa32843d5c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_cc61fad7-4908-40c6-acf5-037dacde60df" xlink:to="loc_srt_RangeAxis_ba80e84d-260e-4116-b40e-92fa32843d5c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_ba80e84d-260e-4116-b40e-92fa32843d5c_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_ba80e84d-260e-4116-b40e-92fa32843d5c" xlink:to="loc_srt_RangeMember_ba80e84d-260e-4116-b40e-92fa32843d5c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_139297fb-8253-475e-8668-130c4fc0d455" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_ba80e84d-260e-4116-b40e-92fa32843d5c" xlink:to="loc_srt_RangeMember_139297fb-8253-475e-8668-130c4fc0d455" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_3822c86f-05e4-4bcc-bcf4-2ab31b151faf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_139297fb-8253-475e-8668-130c4fc0d455" xlink:to="loc_srt_MinimumMember_3822c86f-05e4-4bcc-bcf4-2ab31b151faf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_8fb16a38-d673-4d13-a113-f2d6d73cdef5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_139297fb-8253-475e-8668-130c4fc0d455" xlink:to="loc_srt_MaximumMember_8fb16a38-d673-4d13-a113-f2d6d73cdef5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_7f6bee63-4ecb-4b64-965b-2a34f2b992ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c562b49b-ce1e-43c0-be4d-72d478aa7647" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_7f6bee63-4ecb-4b64-965b-2a34f2b992ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_8cad6669-c30a-42d2-933f-31ccc5ad8b49" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c562b49b-ce1e-43c0-be4d-72d478aa7647" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_8cad6669-c30a-42d2-933f-31ccc5ad8b49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_836e6576-512c-4465-9de9-75c5bcef99e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c562b49b-ce1e-43c0-be4d-72d478aa7647" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_836e6576-512c-4465-9de9-75c5bcef99e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_82a5dc03-7f85-44c6-b16e-7a27623e2d3f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c562b49b-ce1e-43c0-be4d-72d478aa7647" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_82a5dc03-7f85-44c6-b16e-7a27623e2d3f" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/PropertyPlantandEquipmentScheduleofDepreciationExpenseDetails" xlink:type="simple" xlink:href="band-20251231.xsd#PropertyPlantandEquipmentScheduleofDepreciationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/PropertyPlantandEquipmentScheduleofDepreciationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_9e7da46a-f51e-49da-ad89-b1d5d8cb6d98" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_12ad1694-115e-4831-ab1e-c8e53aa02fb3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_9e7da46a-f51e-49da-ad89-b1d5d8cb6d98" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_12ad1694-115e-4831-ab1e-c8e53aa02fb3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_a97af143-3d05-4c83-bb12-80808457e077" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_12ad1694-115e-4831-ab1e-c8e53aa02fb3" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_a97af143-3d05-4c83-bb12-80808457e077" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_a97af143-3d05-4c83-bb12-80808457e077_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_a97af143-3d05-4c83-bb12-80808457e077" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_a97af143-3d05-4c83-bb12-80808457e077_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_4a494df4-a276-4149-9fec-cf14341e7fe1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_a97af143-3d05-4c83-bb12-80808457e077" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_4a494df4-a276-4149-9fec-cf14341e7fe1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_ab1e6ec8-bcbe-4f28-bc4b-0a136d9ed9ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_4a494df4-a276-4149-9fec-cf14341e7fe1" xlink:to="loc_us-gaap_CostOfSalesMember_ab1e6ec8-bcbe-4f28-bc4b-0a136d9ed9ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_cd870773-6bae-411d-b803-1ec3f7d41097" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_4a494df4-a276-4149-9fec-cf14341e7fe1" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_cd870773-6bae-411d-b803-1ec3f7d41097" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_b9d7a071-1905-434a-b63d-83634a3457da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_4a494df4-a276-4149-9fec-cf14341e7fe1" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_b9d7a071-1905-434a-b63d-83634a3457da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_c2004a50-b5d8-4245-bf9f-afc1f66d7f10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_4a494df4-a276-4149-9fec-cf14341e7fe1" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_c2004a50-b5d8-4245-bf9f-afc1f66d7f10" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_08c5f4db-4e77-4a28-af80-27e323eefba5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_9e7da46a-f51e-49da-ad89-b1d5d8cb6d98" xlink:to="loc_us-gaap_Depreciation_08c5f4db-4e77-4a28-af80-27e323eefba5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/GoodwillandIntangibleAssetsScheduleofIntangibleAssetsNetDetails" xlink:type="simple" xlink:href="band-20251231.xsd#GoodwillandIntangibleAssetsScheduleofIntangibleAssetsNetDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/GoodwillandIntangibleAssetsScheduleofIntangibleAssetsNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_15681596-4b06-4fb9-9966-9767188eef51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_2c1b899d-cc35-4e31-b46b-3f10e485439f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_15681596-4b06-4fb9-9966-9767188eef51" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_2c1b899d-cc35-4e31-b46b-3f10e485439f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_29902b07-4245-4d8d-b73c-f02b61c1d476" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_2c1b899d-cc35-4e31-b46b-3f10e485439f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_29902b07-4245-4d8d-b73c-f02b61c1d476" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_29902b07-4245-4d8d-b73c-f02b61c1d476_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_29902b07-4245-4d8d-b73c-f02b61c1d476" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_29902b07-4245-4d8d-b73c-f02b61c1d476_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f30d9de4-81f0-42e1-a703-7593c1d49fbf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_29902b07-4245-4d8d-b73c-f02b61c1d476" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f30d9de4-81f0-42e1-a703-7593c1d49fbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_d7cc8849-b076-4843-b621-a5efda025422" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f30d9de4-81f0-42e1-a703-7593c1d49fbf" xlink:to="loc_us-gaap_CustomerRelationshipsMember_d7cc8849-b076-4843-b621-a5efda025422" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_1389c081-168c-4c69-beb4-46cda8976b19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f30d9de4-81f0-42e1-a703-7593c1d49fbf" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_1389c081-168c-4c69-beb4-46cda8976b19" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_bf6f63d6-2adb-4423-9d3b-99cbd0a2d193" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f30d9de4-81f0-42e1-a703-7593c1d49fbf" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_bf6f63d6-2adb-4423-9d3b-99cbd0a2d193" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_992ed25d-c947-4ab0-a95c-9710c5418cb0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_2c1b899d-cc35-4e31-b46b-3f10e485439f" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_992ed25d-c947-4ab0-a95c-9710c5418cb0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_992ed25d-c947-4ab0-a95c-9710c5418cb0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_992ed25d-c947-4ab0-a95c-9710c5418cb0" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_992ed25d-c947-4ab0-a95c-9710c5418cb0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_eea2d705-d22d-4ddc-b6d3-7255dc44d0de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_992ed25d-c947-4ab0-a95c-9710c5418cb0" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_eea2d705-d22d-4ddc-b6d3-7255dc44d0de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LicensingAgreementsMember_06d3f2f3-8df0-4e98-9489-e722f16af4af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LicensingAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_eea2d705-d22d-4ddc-b6d3-7255dc44d0de" xlink:to="loc_us-gaap_LicensingAgreementsMember_06d3f2f3-8df0-4e98-9489-e722f16af4af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22ebdad6-38fe-454a-8033-8dd823b0e30e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_15681596-4b06-4fb9-9966-9767188eef51" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22ebdad6-38fe-454a-8033-8dd823b0e30e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_5f85e620-efaa-4777-8ffd-4be0a99a7350" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22ebdad6-38fe-454a-8033-8dd823b0e30e" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_5f85e620-efaa-4777-8ffd-4be0a99a7350" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_bbee4b84-e3f1-4d47-ab0a-45001fd448b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22ebdad6-38fe-454a-8033-8dd823b0e30e" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_bbee4b84-e3f1-4d47-ab0a-45001fd448b6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_e7441fff-e52f-4a95-a7c6-6e9eff53898f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22ebdad6-38fe-454a-8033-8dd823b0e30e" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_e7441fff-e52f-4a95-a7c6-6e9eff53898f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_31e1cfd9-9c1d-485b-90e8-744581b1527d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22ebdad6-38fe-454a-8033-8dd823b0e30e" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_31e1cfd9-9c1d-485b-90e8-744581b1527d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_1fd16ef0-d9e4-427a-8c03-c10d165e64de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22ebdad6-38fe-454a-8033-8dd823b0e30e" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_1fd16ef0-d9e4-427a-8c03-c10d165e64de" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_bb7bf564-99e2-4d33-bd3a-c923adf91833" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22ebdad6-38fe-454a-8033-8dd823b0e30e" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_bb7bf564-99e2-4d33-bd3a-c923adf91833" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/GoodwillandIntangibleAssetsScheduleofAmortizationExpenseDetails" xlink:type="simple" xlink:href="band-20251231.xsd#GoodwillandIntangibleAssetsScheduleofAmortizationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/GoodwillandIntangibleAssetsScheduleofAmortizationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_ce293598-6d48-4969-8119-1dc42a8a8b46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_ac251430-cc74-4b6d-a81b-dbd60fdf80e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_ce293598-6d48-4969-8119-1dc42a8a8b46" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_ac251430-cc74-4b6d-a81b-dbd60fdf80e0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_46cbdd46-e1cd-4a0a-8eb5-b2429b197fe4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_ac251430-cc74-4b6d-a81b-dbd60fdf80e0" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_46cbdd46-e1cd-4a0a-8eb5-b2429b197fe4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_46cbdd46-e1cd-4a0a-8eb5-b2429b197fe4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_46cbdd46-e1cd-4a0a-8eb5-b2429b197fe4" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_46cbdd46-e1cd-4a0a-8eb5-b2429b197fe4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_04163890-b364-4efa-b8c6-9c6c1e228569" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_46cbdd46-e1cd-4a0a-8eb5-b2429b197fe4" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_04163890-b364-4efa-b8c6-9c6c1e228569" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_0ee362a9-6036-4faf-8170-1fafefe2ed47" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_04163890-b364-4efa-b8c6-9c6c1e228569" xlink:to="loc_us-gaap_CostOfSalesMember_0ee362a9-6036-4faf-8170-1fafefe2ed47" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_f9f4436c-bb29-4edc-9515-2acc3a61188d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_04163890-b364-4efa-b8c6-9c6c1e228569" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_f9f4436c-bb29-4edc-9515-2acc3a61188d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_67287476-3b4b-4d7a-a043-c21c2f49f6a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_ce293598-6d48-4969-8119-1dc42a8a8b46" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_67287476-3b4b-4d7a-a043-c21c2f49f6a5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtRevolvingCreditFacilityDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DebtRevolvingCreditFacilityDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtRevolvingCreditFacilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_d5825c8b-69f8-4eb3-a8e6-e549c279fb88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:to="loc_us-gaap_CreditFacilityAxis_d5825c8b-69f8-4eb3-a8e6-e549c279fb88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_d5825c8b-69f8-4eb3-a8e6-e549c279fb88_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_d5825c8b-69f8-4eb3-a8e6-e549c279fb88" xlink:to="loc_us-gaap_CreditFacilityDomain_d5825c8b-69f8-4eb3-a8e6-e549c279fb88_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_1c873bdc-cd60-48ae-9b7a-431bb655c961" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_d5825c8b-69f8-4eb3-a8e6-e549c279fb88" xlink:to="loc_us-gaap_CreditFacilityDomain_1c873bdc-cd60-48ae-9b7a-431bb655c961" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_3b6ed78c-3a51-4dbf-9d55-f90376049eb0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_1c873bdc-cd60-48ae-9b7a-431bb655c961" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_3b6ed78c-3a51-4dbf-9d55-f90376049eb0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_57e0a0ec-a5d2-409e-b6ce-c21db50bc313" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_1c873bdc-cd60-48ae-9b7a-431bb655c961" xlink:to="loc_us-gaap_LetterOfCreditMember_57e0a0ec-a5d2-409e-b6ce-c21db50bc313" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BridgeLoanMember_9a9001c9-225e-4645-ad17-20358dd92ef4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BridgeLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_1c873bdc-cd60-48ae-9b7a-431bb655c961" xlink:to="loc_us-gaap_BridgeLoanMember_9a9001c9-225e-4645-ad17-20358dd92ef4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_fb2844eb-cac8-4b39-a070-97d37d2f89f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:to="loc_us-gaap_DebtInstrumentAxis_fb2844eb-cac8-4b39-a070-97d37d2f89f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_fb2844eb-cac8-4b39-a070-97d37d2f89f4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_fb2844eb-cac8-4b39-a070-97d37d2f89f4" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_fb2844eb-cac8-4b39-a070-97d37d2f89f4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_fc14f9e9-6fbf-4c1b-b0dd-7bf65ec08aa9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_fb2844eb-cac8-4b39-a070-97d37d2f89f4" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_fc14f9e9-6fbf-4c1b-b0dd-7bf65ec08aa9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_BankOfAmericaCreditAgreementMember_cea9125e-4ce2-420c-807b-e52acb3b1a37" xlink:href="band-20251231.xsd#band_BankOfAmericaCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_fc14f9e9-6fbf-4c1b-b0dd-7bf65ec08aa9" xlink:to="loc_band_BankOfAmericaCreditAgreementMember_cea9125e-4ce2-420c-807b-e52acb3b1a37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_c6068054-2164-42d5-97f4-b43e69bc50cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_c6068054-2164-42d5-97f4-b43e69bc50cf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_c6068054-2164-42d5-97f4-b43e69bc50cf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c6068054-2164-42d5-97f4-b43e69bc50cf" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_c6068054-2164-42d5-97f4-b43e69bc50cf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_95ceaf80-6c5c-434a-8b02-7c37ff586124" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c6068054-2164-42d5-97f4-b43e69bc50cf" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_95ceaf80-6c5c-434a-8b02-7c37ff586124" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_5742cb89-6575-4357-b5e5-e4b9507c10c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_95ceaf80-6c5c-434a-8b02-7c37ff586124" xlink:to="loc_us-gaap_LineOfCreditMember_5742cb89-6575-4357-b5e5-e4b9507c10c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_442c997c-4f57-4297-b740-ddb601057c10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:to="loc_us-gaap_VariableRateAxis_442c997c-4f57-4297-b740-ddb601057c10" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_442c997c-4f57-4297-b740-ddb601057c10_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_442c997c-4f57-4297-b740-ddb601057c10" xlink:to="loc_us-gaap_VariableRateDomain_442c997c-4f57-4297-b740-ddb601057c10_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_436b61d4-7f12-4ea5-98e6-d80a4ae47dc3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_442c997c-4f57-4297-b740-ddb601057c10" xlink:to="loc_us-gaap_VariableRateDomain_436b61d4-7f12-4ea5-98e6-d80a4ae47dc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_194f6247-d5ee-4b24-98bf-a51950ab059d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_436b61d4-7f12-4ea5-98e6-d80a4ae47dc3" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_194f6247-d5ee-4b24-98bf-a51950ab059d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_1d6885ee-2cc5-4309-8d31-2af00e054bf4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_436b61d4-7f12-4ea5-98e6-d80a4ae47dc3" xlink:to="loc_us-gaap_BaseRateMember_1d6885ee-2cc5-4309-8d31-2af00e054bf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_af6ff785-3ef5-44ac-9b65-116e94ebdc44" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:to="loc_srt_RangeAxis_af6ff785-3ef5-44ac-9b65-116e94ebdc44" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_af6ff785-3ef5-44ac-9b65-116e94ebdc44_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_af6ff785-3ef5-44ac-9b65-116e94ebdc44" xlink:to="loc_srt_RangeMember_af6ff785-3ef5-44ac-9b65-116e94ebdc44_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_63892d68-5fd2-4990-89dd-1cb0a02d8a71" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_af6ff785-3ef5-44ac-9b65-116e94ebdc44" xlink:to="loc_srt_RangeMember_63892d68-5fd2-4990-89dd-1cb0a02d8a71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_fce6dbd0-7351-48cd-a523-9016a07fb20a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_63892d68-5fd2-4990-89dd-1cb0a02d8a71" xlink:to="loc_srt_MinimumMember_fce6dbd0-7351-48cd-a523-9016a07fb20a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_73a0bf97-3319-46b3-994a-cfc3f2aaf197" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_63892d68-5fd2-4990-89dd-1cb0a02d8a71" xlink:to="loc_srt_MaximumMember_73a0bf97-3319-46b3-994a-cfc3f2aaf197" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_f3415411-cd47-4198-a028-d6806a7cc927" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_f3415411-cd47-4198-a028-d6806a7cc927" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_f3415411-cd47-4198-a028-d6806a7cc927_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_f3415411-cd47-4198-a028-d6806a7cc927" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_f3415411-cd47-4198-a028-d6806a7cc927_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_215fb84e-9095-4e64-b757-ff72cfd60c19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_f3415411-cd47-4198-a028-d6806a7cc927" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_215fb84e-9095-4e64-b757-ff72cfd60c19" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_d1a5f594-6b41-4339-a5a4-146c0ca13e95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_215fb84e-9095-4e64-b757-ff72cfd60c19" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_d1a5f594-6b41-4339-a5a4-146c0ca13e95" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodTwoMember_9ebaf780-3467-439d-864e-4bdfe096d0e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_215fb84e-9095-4e64-b757-ff72cfd60c19" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodTwoMember_9ebaf780-3467-439d-864e-4bdfe096d0e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_f3b086fd-9edb-4e92-b568-2f528de17ead" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:to="loc_us-gaap_StatementClassOfStockAxis_f3b086fd-9edb-4e92-b568-2f528de17ead" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f3b086fd-9edb-4e92-b568-2f528de17ead_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f3b086fd-9edb-4e92-b568-2f528de17ead" xlink:to="loc_us-gaap_ClassOfStockDomain_f3b086fd-9edb-4e92-b568-2f528de17ead_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f8f2a9a0-6db1-4a2f-9d73-433fb55b47a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f3b086fd-9edb-4e92-b568-2f528de17ead" xlink:to="loc_us-gaap_ClassOfStockDomain_f8f2a9a0-6db1-4a2f-9d73-433fb55b47a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_VotingCommonStockMember_d25f596f-bebe-458d-aff8-3178dff8524d" xlink:href="band-20251231.xsd#band_VotingCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f8f2a9a0-6db1-4a2f-9d73-433fb55b47a8" xlink:to="loc_band_VotingCommonStockMember_d25f596f-bebe-458d-aff8-3178dff8524d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonvotingCommonStockMember_0524d4b0-c8b9-4b7b-8ad0-20c5a1eefb37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonvotingCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f8f2a9a0-6db1-4a2f-9d73-433fb55b47a8" xlink:to="loc_us-gaap_NonvotingCommonStockMember_0524d4b0-c8b9-4b7b-8ad0-20c5a1eefb37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_703f2127-95bd-4e37-b02c-5946c3cfd945" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_475d0d16-786c-44a5-8245-fe8e04ea2c4d" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_703f2127-95bd-4e37-b02c-5946c3cfd945" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_703f2127-95bd-4e37-b02c-5946c3cfd945_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_703f2127-95bd-4e37-b02c-5946c3cfd945" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_703f2127-95bd-4e37-b02c-5946c3cfd945_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_f3001bf0-e00e-4597-aae9-91c7843f23ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_703f2127-95bd-4e37-b02c-5946c3cfd945" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_f3001bf0-e00e-4597-aae9-91c7843f23ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_2b7aab4d-4388-4ad3-b4e1-48ba2ff4e2a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_f3001bf0-e00e-4597-aae9-91c7843f23ec" xlink:to="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_2b7aab4d-4388-4ad3-b4e1-48ba2ff4e2a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentAssetsMember_ba3c7c7c-ff99-4715-a1d1-78d85779090c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoncurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_f3001bf0-e00e-4597-aae9-91c7843f23ec" xlink:to="loc_us-gaap_OtherNoncurrentAssetsMember_ba3c7c7c-ff99-4715-a1d1-78d85779090c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_a3bcf861-33c9-4554-84e7-0858fcc1cf54" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_a3bcf861-33c9-4554-84e7-0858fcc1cf54" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8ed6f106-71cc-4405-8223-4765ec0972ab" xlink:href="band-20251231.xsd#band_LineOfCreditFacilityAccordionFeatureIncreaseLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8ed6f106-71cc-4405-8223-4765ec0972ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_LineOfCreditFacilityDaysPriorToMaturityDate_65e65688-75ea-48ca-a7b0-cf3d2a2a023e" xlink:href="band-20251231.xsd#band_LineOfCreditFacilityDaysPriorToMaturityDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_LineOfCreditFacilityDaysPriorToMaturityDate_65e65688-75ea-48ca-a7b0-cf3d2a2a023e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentCovenantLeverageRatioMaximumAllowed_f55f1afe-2198-4ae1-b843-3f4139594469" xlink:href="band-20251231.xsd#band_DebtInstrumentCovenantLeverageRatioMaximumAllowed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_DebtInstrumentCovenantLeverageRatioMaximumAllowed_f55f1afe-2198-4ae1-b843-3f4139594469" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentCovenantFixedChargeRatioMinimum_5773a859-ca89-4adf-bced-08514183cfff" xlink:href="band-20251231.xsd#band_DebtInstrumentCovenantFixedChargeRatioMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_DebtInstrumentCovenantFixedChargeRatioMinimum_5773a859-ca89-4adf-bced-08514183cfff" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentCovenantProFormaLeverageRatioMinimum_220db91d-3fde-4890-b300-ba88f46641a0" xlink:href="band-20251231.xsd#band_DebtInstrumentCovenantProFormaLeverageRatioMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_DebtInstrumentCovenantProFormaLeverageRatioMinimum_220db91d-3fde-4890-b300-ba88f46641a0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_LineOfCreditFacilityCovenantConsecutiveQuarters_f850bcdb-7710-413d-8003-97527a00cc2e" xlink:href="band-20251231.xsd#band_LineOfCreditFacilityCovenantConsecutiveQuarters"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_LineOfCreditFacilityCovenantConsecutiveQuarters_f850bcdb-7710-413d-8003-97527a00cc2e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentCovenantProFormaTotalLeverageRatioMaximumAllowed_bcf9cb57-3239-43f8-b459-8436544a1835" xlink:href="band-20251231.xsd#band_DebtInstrumentCovenantProFormaTotalLeverageRatioMaximumAllowed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_DebtInstrumentCovenantProFormaTotalLeverageRatioMaximumAllowed_bcf9cb57-3239-43f8-b459-8436544a1835" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentAdditionalBasisSpreadOnVariableRate_17166db1-9581-49a5-a2aa-8f1ec6a4a928" xlink:href="band-20251231.xsd#band_DebtInstrumentAdditionalBasisSpreadOnVariableRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_DebtInstrumentAdditionalBasisSpreadOnVariableRate_17166db1-9581-49a5-a2aa-8f1ec6a4a928" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_05156e69-a15d-46ee-9731-dd4e4415cf2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_05156e69-a15d-46ee-9731-dd4e4415cf2b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_650ed9ec-478d-4491-868a-8ac88e340f10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_650ed9ec-478d-4491-868a-8ac88e340f10" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentCollateralEquityInterestsPledgedPercentageOfCapitalStock_9fc05b58-9705-4158-930e-9eb433c7a479" xlink:href="band-20251231.xsd#band_DebtInstrumentCollateralEquityInterestsPledgedPercentageOfCapitalStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_band_DebtInstrumentCollateralEquityInterestsPledgedPercentageOfCapitalStock_9fc05b58-9705-4158-930e-9eb433c7a479" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_d6e24de5-d137-43a3-8cbf-5e184136c2e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_d6e24de5-d137-43a3-8cbf-5e184136c2e2" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_a061816e-601c-4129-83c0-de8863449545" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_us-gaap_LineOfCredit_a061816e-601c-4129-83c0-de8863449545" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_e5e104dd-951d-4ac7-932f-71fc9c7feeb2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4f745c9-46dc-4259-8e25-905576679f8d" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_e5e104dd-951d-4ac7-932f-71fc9c7feeb2" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/Debt2026ConvertibleNotesDetails" xlink:type="simple" xlink:href="band-20251231.xsd#Debt2026ConvertibleNotesDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/Debt2026ConvertibleNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_c638d21c-bb5d-4b6d-909e-5349c324f4da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_DebtInstrumentTable_c638d21c-bb5d-4b6d-909e-5349c324f4da" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_b4aaa274-ee0b-4694-9668-59be4c360b33" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c638d21c-bb5d-4b6d-909e-5349c324f4da" xlink:to="loc_us-gaap_DebtInstrumentAxis_b4aaa274-ee0b-4694-9668-59be4c360b33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_b4aaa274-ee0b-4694-9668-59be4c360b33_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_b4aaa274-ee0b-4694-9668-59be4c360b33" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_b4aaa274-ee0b-4694-9668-59be4c360b33_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_1b53210f-6935-406b-a785-77a48a3d51c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_b4aaa274-ee0b-4694-9668-59be4c360b33" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_1b53210f-6935-406b-a785-77a48a3d51c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_09f380b1-838f-4a26-a5e2-f0b91114b070" xlink:href="band-20251231.xsd#band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_1b53210f-6935-406b-a785-77a48a3d51c5" xlink:to="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_09f380b1-838f-4a26-a5e2-f0b91114b070" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_ae36549a-7dd6-445c-a2c7-e617d4126524" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c638d21c-bb5d-4b6d-909e-5349c324f4da" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_ae36549a-7dd6-445c-a2c7-e617d4126524" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_ae36549a-7dd6-445c-a2c7-e617d4126524_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_ae36549a-7dd6-445c-a2c7-e617d4126524" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_ae36549a-7dd6-445c-a2c7-e617d4126524_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9b12855c-d085-41d0-9af9-7411e00f9abc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_ae36549a-7dd6-445c-a2c7-e617d4126524" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9b12855c-d085-41d0-9af9-7411e00f9abc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_ae1cf694-9d37-47fb-9073-9f508dc84374" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9b12855c-d085-41d0-9af9-7411e00f9abc" xlink:to="loc_us-gaap_ConvertibleDebtMember_ae1cf694-9d37-47fb-9073-9f508dc84374" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_2cac57db-688b-4550-af5c-f5c462e54222" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c638d21c-bb5d-4b6d-909e-5349c324f4da" xlink:to="loc_us-gaap_StatementClassOfStockAxis_2cac57db-688b-4550-af5c-f5c462e54222" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_2cac57db-688b-4550-af5c-f5c462e54222_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_2cac57db-688b-4550-af5c-f5c462e54222" xlink:to="loc_us-gaap_ClassOfStockDomain_2cac57db-688b-4550-af5c-f5c462e54222_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_a0da7562-3cd2-4608-be38-1dfe29e1c7bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_2cac57db-688b-4550-af5c-f5c462e54222" xlink:to="loc_us-gaap_ClassOfStockDomain_a0da7562-3cd2-4608-be38-1dfe29e1c7bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_88e76a4b-1c7b-4153-95a3-42a2da4205b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_a0da7562-3cd2-4608-be38-1dfe29e1c7bb" xlink:to="loc_us-gaap_CommonClassAMember_88e76a4b-1c7b-4153-95a3-42a2da4205b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_8e16e5c1-579d-440e-8210-615a43603ef4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_8e16e5c1-579d-440e-8210-615a43603ef4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_3172a4b3-7cf5-4d34-817f-cd97dd67dd2f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_3172a4b3-7cf5-4d34-817f-cd97dd67dd2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet_0a4dc99f-2d94-4953-b388-13bac53af958" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet_0a4dc99f-2d94-4953-b388-13bac53af958" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_18bc8894-9b97-4d07-bba0-2c73ef75d89c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_18bc8894-9b97-4d07-bba0-2c73ef75d89c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_f980c6bb-89fe-4def-a0ff-4e4230f5615b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_f980c6bb-89fe-4def-a0ff-4e4230f5615b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_d772f953-0cfb-457a-b6e9-ebf2bf1f1914" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_d772f953-0cfb-457a-b6e9-ebf2bf1f1914" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_b82e093b-c826-4b68-bbaa-ec245a3d87dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_b82e093b-c826-4b68-bbaa-ec245a3d87dd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRepurchasedFaceAmount_936d59f7-0637-4fda-8afc-542ea751aa71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRepurchasedFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_DebtInstrumentRepurchasedFaceAmount_936d59f7-0637-4fda-8afc-542ea751aa71" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_363dc2b3-0067-4b11-b079-da1a5dcc92d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_363dc2b3-0067-4b11-b079-da1a5dcc92d7" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_2853bc00-e576-443a-9c30-358229fb7a9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f535185c-93c7-492b-84b9-dbb199ed44bb" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_2853bc00-e576-443a-9c30-358229fb7a9d" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/Debt2028ConvertibleNotesDetails" xlink:type="simple" xlink:href="band-20251231.xsd#Debt2028ConvertibleNotesDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/Debt2028ConvertibleNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_21d0838d-eddd-4148-838b-9151fddcadc9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:to="loc_us-gaap_DebtInstrumentTable_21d0838d-eddd-4148-838b-9151fddcadc9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_21707615-89a0-402b-8dcd-254d7e43d7d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_21d0838d-eddd-4148-838b-9151fddcadc9" xlink:to="loc_us-gaap_DebtInstrumentAxis_21707615-89a0-402b-8dcd-254d7e43d7d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_21707615-89a0-402b-8dcd-254d7e43d7d9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_21707615-89a0-402b-8dcd-254d7e43d7d9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_21707615-89a0-402b-8dcd-254d7e43d7d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_260584ae-b194-456e-bc38-5d601b04029e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_21707615-89a0-402b-8dcd-254d7e43d7d9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_260584ae-b194-456e-bc38-5d601b04029e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_bdea9077-905e-4b61-9a23-3bcffb2aaa76" xlink:href="band-20251231.xsd#band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_260584ae-b194-456e-bc38-5d601b04029e" xlink:to="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_bdea9077-905e-4b61-9a23-3bcffb2aaa76" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_968b1ce1-ec69-40da-8e1c-bc92b8dd0b93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_21d0838d-eddd-4148-838b-9151fddcadc9" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_968b1ce1-ec69-40da-8e1c-bc92b8dd0b93" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_968b1ce1-ec69-40da-8e1c-bc92b8dd0b93_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_968b1ce1-ec69-40da-8e1c-bc92b8dd0b93" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_968b1ce1-ec69-40da-8e1c-bc92b8dd0b93_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_b3912931-9b81-460c-a066-f68848cac7a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_968b1ce1-ec69-40da-8e1c-bc92b8dd0b93" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_b3912931-9b81-460c-a066-f68848cac7a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_5ed8a5ea-77ad-4dd0-9c91-611840e04a4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b3912931-9b81-460c-a066-f68848cac7a5" xlink:to="loc_us-gaap_ConvertibleDebtMember_5ed8a5ea-77ad-4dd0-9c91-611840e04a4e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_0c842137-7550-409f-bf40-426c3a2c69ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_21d0838d-eddd-4148-838b-9151fddcadc9" xlink:to="loc_us-gaap_StatementClassOfStockAxis_0c842137-7550-409f-bf40-426c3a2c69ff" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_0c842137-7550-409f-bf40-426c3a2c69ff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0c842137-7550-409f-bf40-426c3a2c69ff" xlink:to="loc_us-gaap_ClassOfStockDomain_0c842137-7550-409f-bf40-426c3a2c69ff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f5c2d691-0b19-4df6-b509-022659e30f12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0c842137-7550-409f-bf40-426c3a2c69ff" xlink:to="loc_us-gaap_ClassOfStockDomain_f5c2d691-0b19-4df6-b509-022659e30f12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_fa1b129c-47c6-4916-a8b3-9f80ee21dfc6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f5c2d691-0b19-4df6-b509-022659e30f12" xlink:to="loc_us-gaap_CommonClassAMember_fa1b129c-47c6-4916-a8b3-9f80ee21dfc6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_f8754b7f-f058-4c34-9003-b6b49a4fa5be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_f8754b7f-f058-4c34-9003-b6b49a4fa5be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_5205fa65-3f29-438f-8a6f-622b57b73eba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_5205fa65-3f29-438f-8a6f-622b57b73eba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet_aa105a0a-f3d4-472b-a557-4e0aea9f1d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet_aa105a0a-f3d4-472b-a557-4e0aea9f1d4c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_b8234584-0992-4449-947a-e56e22aabd75" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_b8234584-0992-4449-947a-e56e22aabd75" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_4d72d948-7703-4191-86e6-166b115c7219" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_4d72d948-7703-4191-86e6-166b115c7219" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_40f44465-c4a6-468b-ab7e-1abf1f3d0a40" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_40f44465-c4a6-468b-ab7e-1abf1f3d0a40" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_0d63a5a0-9174-4da3-8848-fb882fbf7c82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ecbc07ef-13cf-4f67-b230-b00b784f5bbe" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_0d63a5a0-9174-4da3-8848-fb882fbf7c82" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtOtherTermsoftheConvertibleNotesDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DebtOtherTermsoftheConvertibleNotesDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtOtherTermsoftheConvertibleNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_c029dd2f-4341-436c-834d-bab691cfa864" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_5b83b13b-b081-418f-9abf-88aecdd21e22" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c029dd2f-4341-436c-834d-bab691cfa864" xlink:to="loc_us-gaap_DebtInstrumentTable_5b83b13b-b081-418f-9abf-88aecdd21e22" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_f8ab557b-b469-47a6-9a41-e9d9aff5798f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5b83b13b-b081-418f-9abf-88aecdd21e22" xlink:to="loc_us-gaap_DebtInstrumentAxis_f8ab557b-b469-47a6-9a41-e9d9aff5798f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_f8ab557b-b469-47a6-9a41-e9d9aff5798f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_f8ab557b-b469-47a6-9a41-e9d9aff5798f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_f8ab557b-b469-47a6-9a41-e9d9aff5798f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_f660db2d-283b-4edb-bf95-1a12196c5f72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_f8ab557b-b469-47a6-9a41-e9d9aff5798f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_f660db2d-283b-4edb-bf95-1a12196c5f72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_64553b15-eee7-45cf-b54f-d9c5d55ff016" xlink:href="band-20251231.xsd#band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f660db2d-283b-4edb-bf95-1a12196c5f72" xlink:to="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_64553b15-eee7-45cf-b54f-d9c5d55ff016" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_924f28e0-3756-4396-ab7d-c41f144f2f5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5b83b13b-b081-418f-9abf-88aecdd21e22" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_924f28e0-3756-4396-ab7d-c41f144f2f5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_924f28e0-3756-4396-ab7d-c41f144f2f5b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_924f28e0-3756-4396-ab7d-c41f144f2f5b" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_924f28e0-3756-4396-ab7d-c41f144f2f5b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_e813e8e7-07d1-402d-8fc4-2571ba709a3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_924f28e0-3756-4396-ab7d-c41f144f2f5b" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_e813e8e7-07d1-402d-8fc4-2571ba709a3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_63293155-d856-4bbc-9dd8-182f53871c71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_e813e8e7-07d1-402d-8fc4-2571ba709a3c" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_63293155-d856-4bbc-9dd8-182f53871c71" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_80fa658c-ed06-475f-b118-54f119e0aa64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5b83b13b-b081-418f-9abf-88aecdd21e22" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_80fa658c-ed06-475f-b118-54f119e0aa64" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_80fa658c-ed06-475f-b118-54f119e0aa64_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_80fa658c-ed06-475f-b118-54f119e0aa64" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_80fa658c-ed06-475f-b118-54f119e0aa64_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_91ac90b9-d3e5-469a-a3b8-afebb258bb44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_80fa658c-ed06-475f-b118-54f119e0aa64" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_91ac90b9-d3e5-469a-a3b8-afebb258bb44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_5987ac66-6907-42cd-aa9a-1cc28f34a2b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_91ac90b9-d3e5-469a-a3b8-afebb258bb44" xlink:to="loc_us-gaap_ConvertibleDebtMember_5987ac66-6907-42cd-aa9a-1cc28f34a2b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_27d11ac9-5ae9-4b0f-939c-01e9994cf425" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5b83b13b-b081-418f-9abf-88aecdd21e22" xlink:to="loc_us-gaap_StatementClassOfStockAxis_27d11ac9-5ae9-4b0f-939c-01e9994cf425" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_27d11ac9-5ae9-4b0f-939c-01e9994cf425_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_27d11ac9-5ae9-4b0f-939c-01e9994cf425" xlink:to="loc_us-gaap_ClassOfStockDomain_27d11ac9-5ae9-4b0f-939c-01e9994cf425_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_06662f54-0743-4f23-b2f0-56538b142939" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_27d11ac9-5ae9-4b0f-939c-01e9994cf425" xlink:to="loc_us-gaap_ClassOfStockDomain_06662f54-0743-4f23-b2f0-56538b142939" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_2a60bcb7-8300-47f0-b854-5823526db989" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_06662f54-0743-4f23-b2f0-56538b142939" xlink:to="loc_us-gaap_CommonClassAMember_2a60bcb7-8300-47f0-b854-5823526db989" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentRedemptionPeriodTradingDaysToMaturity_acbfb033-f472-4bc1-8aed-69ace851baca" xlink:href="band-20251231.xsd#band_DebtInstrumentRedemptionPeriodTradingDaysToMaturity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c029dd2f-4341-436c-834d-bab691cfa864" xlink:to="loc_band_DebtInstrumentRedemptionPeriodTradingDaysToMaturity_acbfb033-f472-4bc1-8aed-69ace851baca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_59c095c5-38e8-4493-972c-eeed135e1869" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c029dd2f-4341-436c-834d-bab691cfa864" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_59c095c5-38e8-4493-972c-eeed135e1869" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_7eb86301-d018-4d06-8d70-e4d4b3349175" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c029dd2f-4341-436c-834d-bab691cfa864" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_7eb86301-d018-4d06-8d70-e4d4b3349175" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_b976c318-0f2e-47da-a2fe-3c8c41ac051d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c029dd2f-4341-436c-834d-bab691cfa864" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_b976c318-0f2e-47da-a2fe-3c8c41ac051d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_e981025a-5c62-452a-970b-aa9f7aa93015" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c029dd2f-4341-436c-834d-bab691cfa864" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_e981025a-5c62-452a-970b-aa9f7aa93015" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtScheduleofNetCarryingAmountofLiabilityComponentsofConvertibleNotesDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DebtScheduleofNetCarryingAmountofLiabilityComponentsofConvertibleNotesDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtScheduleofNetCarryingAmountofLiabilityComponentsofConvertibleNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_2a96b45f-1609-4e8c-b326-4500e8baad56" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_38548b35-a1ce-43d1-bb03-1dd33fe652b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_2a96b45f-1609-4e8c-b326-4500e8baad56" xlink:to="loc_us-gaap_DebtInstrumentTable_38548b35-a1ce-43d1-bb03-1dd33fe652b7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_d7d8435e-b726-4b10-b98b-cb4723c48f8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_38548b35-a1ce-43d1-bb03-1dd33fe652b7" xlink:to="loc_us-gaap_DebtInstrumentAxis_d7d8435e-b726-4b10-b98b-cb4723c48f8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_d7d8435e-b726-4b10-b98b-cb4723c48f8a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_d7d8435e-b726-4b10-b98b-cb4723c48f8a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_d7d8435e-b726-4b10-b98b-cb4723c48f8a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_8f83b9a2-19f2-481a-aa15-b0ca3c4058b1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_d7d8435e-b726-4b10-b98b-cb4723c48f8a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_8f83b9a2-19f2-481a-aa15-b0ca3c4058b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_8c48ed41-67c6-4282-adac-5563d764f759" xlink:href="band-20251231.xsd#band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_8f83b9a2-19f2-481a-aa15-b0ca3c4058b1" xlink:to="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_8c48ed41-67c6-4282-adac-5563d764f759" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_92582b70-2b85-46fa-89df-70dd3b90707e" xlink:href="band-20251231.xsd#band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_8f83b9a2-19f2-481a-aa15-b0ca3c4058b1" xlink:to="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_92582b70-2b85-46fa-89df-70dd3b90707e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_88acdbe2-73ab-4bd3-831d-b7433de1c5f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_38548b35-a1ce-43d1-bb03-1dd33fe652b7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_88acdbe2-73ab-4bd3-831d-b7433de1c5f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_88acdbe2-73ab-4bd3-831d-b7433de1c5f2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_88acdbe2-73ab-4bd3-831d-b7433de1c5f2" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_88acdbe2-73ab-4bd3-831d-b7433de1c5f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_1b264f57-869f-4dc4-9696-cc5a0966dfec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_88acdbe2-73ab-4bd3-831d-b7433de1c5f2" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_1b264f57-869f-4dc4-9696-cc5a0966dfec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_cf724433-4be8-49a6-8dee-8e2f2a5ecf58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_1b264f57-869f-4dc4-9696-cc5a0966dfec" xlink:to="loc_us-gaap_ConvertibleDebtMember_cf724433-4be8-49a6-8dee-8e2f2a5ecf58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_fe3884fd-39f9-4dbe-a3a8-b230639c6ffe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_2a96b45f-1609-4e8c-b326-4500e8baad56" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_fe3884fd-39f9-4dbe-a3a8-b230639c6ffe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_11c6a40b-6d55-4ec5-8469-bfe0503d29a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_2a96b45f-1609-4e8c-b326-4500e8baad56" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_11c6a40b-6d55-4ec5-8469-bfe0503d29a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_e39045fd-2e7b-4472-ab58-e8c5a66da0f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_2a96b45f-1609-4e8c-b326-4500e8baad56" xlink:to="loc_us-gaap_LongTermDebt_e39045fd-2e7b-4472-ab58-e8c5a66da0f2" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtScheduleofInterestExpenseRecognizedRelatedtoConvertibleNotesDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DebtScheduleofInterestExpenseRecognizedRelatedtoConvertibleNotesDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtScheduleofInterestExpenseRecognizedRelatedtoConvertibleNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_881a2a46-6b74-4ad6-8603-d207663aaacd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_778f8eaf-c4e0-4908-a263-f45f3c6ebd28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_881a2a46-6b74-4ad6-8603-d207663aaacd" xlink:to="loc_us-gaap_DebtInstrumentTable_778f8eaf-c4e0-4908-a263-f45f3c6ebd28" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_57658bb0-bfe2-46c4-b96f-8cab0ec7a4d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_778f8eaf-c4e0-4908-a263-f45f3c6ebd28" xlink:to="loc_us-gaap_DebtInstrumentAxis_57658bb0-bfe2-46c4-b96f-8cab0ec7a4d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_57658bb0-bfe2-46c4-b96f-8cab0ec7a4d3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_57658bb0-bfe2-46c4-b96f-8cab0ec7a4d3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_57658bb0-bfe2-46c4-b96f-8cab0ec7a4d3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_27213de8-17a4-4412-8354-48b00f42fd93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_57658bb0-bfe2-46c4-b96f-8cab0ec7a4d3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_27213de8-17a4-4412-8354-48b00f42fd93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_c0c05ead-9242-40d8-862a-59950ccf0396" xlink:href="band-20251231.xsd#band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_27213de8-17a4-4412-8354-48b00f42fd93" xlink:to="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_c0c05ead-9242-40d8-862a-59950ccf0396" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_7c68127a-81e7-495d-88af-b2e50306a36a" xlink:href="band-20251231.xsd#band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_27213de8-17a4-4412-8354-48b00f42fd93" xlink:to="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_7c68127a-81e7-495d-88af-b2e50306a36a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_2db3efda-92fc-4009-8f4b-322f400cea5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebtExcludingAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_881a2a46-6b74-4ad6-8603-d207663aaacd" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_2db3efda-92fc-4009-8f4b-322f400cea5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfDebtDiscountPremium_b3131327-0c90-4b76-9354-8562fc23c777" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfDebtDiscountPremium"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_881a2a46-6b74-4ad6-8603-d207663aaacd" xlink:to="loc_us-gaap_AmortizationOfDebtDiscountPremium_b3131327-0c90-4b76-9354-8562fc23c777" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_fde75763-233e-438d-a7fd-f7edc6447e5f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_881a2a46-6b74-4ad6-8603-d207663aaacd" xlink:to="loc_us-gaap_InterestExpenseDebt_fde75763-233e-438d-a7fd-f7edc6447e5f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtScheduleofCostsIncurredforCappedCallsRelatedtoConvertibleNotesDetails" xlink:type="simple" xlink:href="band-20251231.xsd#DebtScheduleofCostsIncurredforCappedCallsRelatedtoConvertibleNotesDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtScheduleofCostsIncurredforCappedCallsRelatedtoConvertibleNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_b8354bdb-65cf-4547-b9a6-e449f1df6dba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_d13c37f6-d6f9-45af-a7e8-b2347c274f01" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b8354bdb-65cf-4547-b9a6-e449f1df6dba" xlink:to="loc_us-gaap_DebtInstrumentTable_d13c37f6-d6f9-45af-a7e8-b2347c274f01" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_51147a2c-b640-4955-b1fd-5a5eaf160655" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_d13c37f6-d6f9-45af-a7e8-b2347c274f01" xlink:to="loc_us-gaap_DebtInstrumentAxis_51147a2c-b640-4955-b1fd-5a5eaf160655" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_51147a2c-b640-4955-b1fd-5a5eaf160655_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_51147a2c-b640-4955-b1fd-5a5eaf160655" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_51147a2c-b640-4955-b1fd-5a5eaf160655_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c32b56b5-9f32-4807-b6b7-52c86f03e565" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_51147a2c-b640-4955-b1fd-5a5eaf160655" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c32b56b5-9f32-4807-b6b7-52c86f03e565" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_7581cece-c4d5-4610-8544-5d50f1c2f314" xlink:href="band-20251231.xsd#band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c32b56b5-9f32-4807-b6b7-52c86f03e565" xlink:to="loc_band_ZeroPointTwoFivePercentConvertibleSeniorNotesDue2026Member_7581cece-c4d5-4610-8544-5d50f1c2f314" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_8f0d88cb-290f-40b1-83bb-3d7349443348" xlink:href="band-20251231.xsd#band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c32b56b5-9f32-4807-b6b7-52c86f03e565" xlink:to="loc_band_ZeroPointFiveZeroPercentConvertibleSeniorNotesDue2028Member_8f0d88cb-290f-40b1-83bb-3d7349443348" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_c2155f12-eb28-4fb0-bdf6-e63d92f5dfd7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_d13c37f6-d6f9-45af-a7e8-b2347c274f01" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_c2155f12-eb28-4fb0-bdf6-e63d92f5dfd7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_c2155f12-eb28-4fb0-bdf6-e63d92f5dfd7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c2155f12-eb28-4fb0-bdf6-e63d92f5dfd7" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_c2155f12-eb28-4fb0-bdf6-e63d92f5dfd7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_a8d2965d-af73-4da4-a0cd-3a8c52532d74" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c2155f12-eb28-4fb0-bdf6-e63d92f5dfd7" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_a8d2965d-af73-4da4-a0cd-3a8c52532d74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_831dd465-002b-4a81-80b8-c6f8808badba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_a8d2965d-af73-4da4-a0cd-3a8c52532d74" xlink:to="loc_us-gaap_ConvertibleDebtMember_831dd465-002b-4a81-80b8-c6f8808badba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_b80a2a6d-8023-4856-885a-fdb8ee8ef217" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_d13c37f6-d6f9-45af-a7e8-b2347c274f01" xlink:to="loc_us-gaap_StatementClassOfStockAxis_b80a2a6d-8023-4856-885a-fdb8ee8ef217" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b80a2a6d-8023-4856-885a-fdb8ee8ef217_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b80a2a6d-8023-4856-885a-fdb8ee8ef217" xlink:to="loc_us-gaap_ClassOfStockDomain_b80a2a6d-8023-4856-885a-fdb8ee8ef217_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_7aa1295e-b07a-4af0-8124-14ada2ee6e3f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b80a2a6d-8023-4856-885a-fdb8ee8ef217" xlink:to="loc_us-gaap_ClassOfStockDomain_7aa1295e-b07a-4af0-8124-14ada2ee6e3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_db2a6708-5bd8-4db2-aa90-98c532d312db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_7aa1295e-b07a-4af0-8124-14ada2ee6e3f" xlink:to="loc_us-gaap_CommonClassAMember_db2a6708-5bd8-4db2-aa90-98c532d312db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentConvertibleInitialConversionStrikePrice_834cee3e-43b6-4441-a61f-0b1f23aff642" xlink:href="band-20251231.xsd#band_DebtInstrumentConvertibleInitialConversionStrikePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b8354bdb-65cf-4547-b9a6-e449f1df6dba" xlink:to="loc_band_DebtInstrumentConvertibleInitialConversionStrikePrice_834cee3e-43b6-4441-a61f-0b1f23aff642" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentConvertibleInitialCapPrice_bb22c87a-b4ef-4f53-8ae5-a8e9e5348c92" xlink:href="band-20251231.xsd#band_DebtInstrumentConvertibleInitialCapPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b8354bdb-65cf-4547-b9a6-e449f1df6dba" xlink:to="loc_band_DebtInstrumentConvertibleInitialCapPrice_bb22c87a-b4ef-4f53-8ae5-a8e9e5348c92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CappedCallOptionIssuanceCost_d8a5080a-35e1-4fd0-a6fd-0589f8ec5142" xlink:href="band-20251231.xsd#band_CappedCallOptionIssuanceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b8354bdb-65cf-4547-b9a6-e449f1df6dba" xlink:to="loc_band_CappedCallOptionIssuanceCost_d8a5080a-35e1-4fd0-a6fd-0589f8ec5142" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DebtInstrumentConvertibleConversionCappedCallShares_bc8a8dd8-707d-4f93-aaa9-c5d974af9bbf" xlink:href="band-20251231.xsd#band_DebtInstrumentConvertibleConversionCappedCallShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b8354bdb-65cf-4547-b9a6-e449f1df6dba" xlink:to="loc_band_DebtInstrumentConvertibleConversionCappedCallShares_bc8a8dd8-707d-4f93-aaa9-c5d974af9bbf" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/GeographicInformationScheduleofRevenuebyGeographicRegionDetails" xlink:type="simple" xlink:href="band-20251231.xsd#GeographicInformationScheduleofRevenuebyGeographicRegionDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/GeographicInformationScheduleofRevenuebyGeographicRegionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_c414f832-57f8-4795-be61-7ed70f64d146" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_17db4b33-84d3-4a24-aa32-5c6a10dd84ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_c414f832-57f8-4795-be61-7ed70f64d146" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_17db4b33-84d3-4a24-aa32-5c6a10dd84ff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_e326166a-3506-412d-ba89-d2f9ffe659f4" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_17db4b33-84d3-4a24-aa32-5c6a10dd84ff" xlink:to="loc_srt_StatementGeographicalAxis_e326166a-3506-412d-ba89-d2f9ffe659f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_e326166a-3506-412d-ba89-d2f9ffe659f4_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_e326166a-3506-412d-ba89-d2f9ffe659f4" xlink:to="loc_srt_SegmentGeographicalDomain_e326166a-3506-412d-ba89-d2f9ffe659f4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_e72fc6f1-4b7e-4a10-938b-be42f41bf81d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_e326166a-3506-412d-ba89-d2f9ffe659f4" xlink:to="loc_srt_SegmentGeographicalDomain_e72fc6f1-4b7e-4a10-938b-be42f41bf81d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_51dcd8a2-a676-425b-8d03-a2b60c521f8e" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_e72fc6f1-4b7e-4a10-938b-be42f41bf81d" xlink:to="loc_country_US_51dcd8a2-a676-425b-8d03-a2b60c521f8e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_cc08fd6c-8415-4e17-adbb-c91f75d25b58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_e72fc6f1-4b7e-4a10-938b-be42f41bf81d" xlink:to="loc_us-gaap_NonUsMember_cc08fd6c-8415-4e17-adbb-c91f75d25b58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_1a1b5021-bf46-4256-8038-80026f5f82e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_c414f832-57f8-4795-be61-7ed70f64d146" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_1a1b5021-bf46-4256-8038-80026f5f82e8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/GeographicInformationScheduleofLongLivedAssetsbyGeographicRegionDetails" xlink:type="simple" xlink:href="band-20251231.xsd#GeographicInformationScheduleofLongLivedAssetsbyGeographicRegionDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/GeographicInformationScheduleofLongLivedAssetsbyGeographicRegionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0d3859ba-a570-4f6b-97a5-8ea88b6c18bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_a2b85845-0642-435a-8911-1571c9716e8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0d3859ba-a570-4f6b-97a5-8ea88b6c18bf" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_a2b85845-0642-435a-8911-1571c9716e8d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_ae443476-96fa-46db-b09d-ddb33dab2fb1" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_a2b85845-0642-435a-8911-1571c9716e8d" xlink:to="loc_srt_StatementGeographicalAxis_ae443476-96fa-46db-b09d-ddb33dab2fb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_ae443476-96fa-46db-b09d-ddb33dab2fb1_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_ae443476-96fa-46db-b09d-ddb33dab2fb1" xlink:to="loc_srt_SegmentGeographicalDomain_ae443476-96fa-46db-b09d-ddb33dab2fb1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_661dcdf3-b242-476a-9f9a-0d1cd30e6b96" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_ae443476-96fa-46db-b09d-ddb33dab2fb1" xlink:to="loc_srt_SegmentGeographicalDomain_661dcdf3-b242-476a-9f9a-0d1cd30e6b96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_b808b888-98c1-4fd1-a5c0-c3b42ba73107" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_661dcdf3-b242-476a-9f9a-0d1cd30e6b96" xlink:to="loc_country_US_b808b888-98c1-4fd1-a5c0-c3b42ba73107" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_0a52267a-f05a-45be-810a-45d1c80f12fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_661dcdf3-b242-476a-9f9a-0d1cd30e6b96" xlink:to="loc_us-gaap_NonUsMember_0a52267a-f05a-45be-810a-45d1c80f12fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_f03af019-49db-4867-b5d0-0260ed0618ea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0d3859ba-a570-4f6b-97a5-8ea88b6c18bf" xlink:to="loc_us-gaap_NoncurrentAssets_f03af019-49db-4867-b5d0-0260ed0618ea" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockholdersEquityAdditionalInformationDetails" xlink:type="simple" xlink:href="band-20251231.xsd#StockholdersEquityAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockholdersEquityAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_9e7c158f-54c6-4d53-8fa9-824eeb2b1031" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_a94ede42-9954-47f4-aad8-fae3656ea4c2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_9e7c158f-54c6-4d53-8fa9-824eeb2b1031" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_a94ede42-9954-47f4-aad8-fae3656ea4c2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_f9e6a8aa-140d-4820-904d-0324a2582dac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_a94ede42-9954-47f4-aad8-fae3656ea4c2" xlink:to="loc_us-gaap_StatementClassOfStockAxis_f9e6a8aa-140d-4820-904d-0324a2582dac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f9e6a8aa-140d-4820-904d-0324a2582dac_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f9e6a8aa-140d-4820-904d-0324a2582dac" xlink:to="loc_us-gaap_ClassOfStockDomain_f9e6a8aa-140d-4820-904d-0324a2582dac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_9c9f9641-94f7-4d54-9087-1935f5a401ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f9e6a8aa-140d-4820-904d-0324a2582dac" xlink:to="loc_us-gaap_ClassOfStockDomain_9c9f9641-94f7-4d54-9087-1935f5a401ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_26c05466-dd8d-49cd-a16b-c065af79ba70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9c9f9641-94f7-4d54-9087-1935f5a401ff" xlink:to="loc_us-gaap_CommonClassAMember_26c05466-dd8d-49cd-a16b-c065af79ba70" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_2453e067-fcff-480f-a390-f3779d93c606" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9c9f9641-94f7-4d54-9087-1935f5a401ff" xlink:to="loc_us-gaap_CommonClassBMember_2453e067-fcff-480f-a390-f3779d93c606" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_a595c2de-adba-4b92-9e3e-e6b27b514e21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_9e7c158f-54c6-4d53-8fa9-824eeb2b1031" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_a595c2de-adba-4b92-9e3e-e6b27b514e21" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CommonStockVotingRightsVotesPerShare_e7072372-b9bd-4ff8-a445-c874cb048d37" xlink:href="band-20251231.xsd#band_CommonStockVotingRightsVotesPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_9e7c158f-54c6-4d53-8fa9-824eeb2b1031" xlink:to="loc_band_CommonStockVotingRightsVotesPerShare_e7072372-b9bd-4ff8-a445-c874cb048d37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_bff124e3-5d99-4077-8d7c-bf370e046e0e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockDividendsPerShareDeclared"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_9e7c158f-54c6-4d53-8fa9-824eeb2b1031" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_bff124e3-5d99-4077-8d7c-bf370e046e0e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockholdersEquityScheduleofReservedSharesofCommonStockforIssuanceDetails" xlink:type="simple" xlink:href="band-20251231.xsd#StockholdersEquityScheduleofReservedSharesofCommonStockforIssuanceDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockholdersEquityScheduleofReservedSharesofCommonStockforIssuanceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_2295dd38-f5c7-4079-a448-ae1fb2056eb1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_da016ae2-616a-49de-9c0c-53d5f33c038b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_2295dd38-f5c7-4079-a448-ae1fb2056eb1" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_da016ae2-616a-49de-9c0c-53d5f33c038b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_ffd7881f-b327-484f-b3dc-a1fe6fdf0f92" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_da016ae2-616a-49de-9c0c-53d5f33c038b" xlink:to="loc_us-gaap_AwardTypeAxis_ffd7881f-b327-484f-b3dc-a1fe6fdf0f92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ffd7881f-b327-484f-b3dc-a1fe6fdf0f92_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_ffd7881f-b327-484f-b3dc-a1fe6fdf0f92" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ffd7881f-b327-484f-b3dc-a1fe6fdf0f92_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_69e65b3a-a009-4fb1-af4a-62b93f23cce5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_ffd7881f-b327-484f-b3dc-a1fe6fdf0f92" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_69e65b3a-a009-4fb1-af4a-62b93f23cce5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_3351ca11-52e5-46e4-be83-929620baf5dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_69e65b3a-a009-4fb1-af4a-62b93f23cce5" xlink:to="loc_us-gaap_EmployeeStockOptionMember_3351ca11-52e5-46e4-be83-929620baf5dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_cf0f5938-621e-4590-9d8b-b5eb2987beb3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_69e65b3a-a009-4fb1-af4a-62b93f23cce5" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_cf0f5938-621e-4590-9d8b-b5eb2987beb3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_001363ec-faa9-467f-9895-1054e47dfb11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_69e65b3a-a009-4fb1-af4a-62b93f23cce5" xlink:to="loc_us-gaap_StockCompensationPlanMember_001363ec-faa9-467f-9895-1054e47dfb11" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_ae34900f-01a4-4967-b6ca-b06c9a6c96b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_2295dd38-f5c7-4079-a448-ae1fb2056eb1" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_ae34900f-01a4-4967-b6ca-b06c9a6c96b0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensationAdditionalInformationDetails" xlink:type="simple" xlink:href="band-20251231.xsd#StockBasedCompensationAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensationAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2cb4cdac-3cce-4bf1-8ce2-5fe56b6db2d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2cb4cdac-3cce-4bf1-8ce2-5fe56b6db2d4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_ab7d8cdd-16e4-4833-a9d2-d39fca2bfcd9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2cb4cdac-3cce-4bf1-8ce2-5fe56b6db2d4" xlink:to="loc_us-gaap_AwardTypeAxis_ab7d8cdd-16e4-4833-a9d2-d39fca2bfcd9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ab7d8cdd-16e4-4833-a9d2-d39fca2bfcd9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_ab7d8cdd-16e4-4833-a9d2-d39fca2bfcd9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ab7d8cdd-16e4-4833-a9d2-d39fca2bfcd9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_84fe35ae-49db-432d-b681-185f1e37be13" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_ab7d8cdd-16e4-4833-a9d2-d39fca2bfcd9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_84fe35ae-49db-432d-b681-185f1e37be13" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_774c09d3-7b92-4952-b641-6142381a3d4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_84fe35ae-49db-432d-b681-185f1e37be13" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_774c09d3-7b92-4952-b641-6142381a3d4f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_a59cdc31-e723-407c-95e3-24982f984548" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_84fe35ae-49db-432d-b681-185f1e37be13" xlink:to="loc_us-gaap_EmployeeStockOptionMember_a59cdc31-e723-407c-95e3-24982f984548" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_33ec534e-5d37-42e1-9464-bf34e0d9377e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2cb4cdac-3cce-4bf1-8ce2-5fe56b6db2d4" xlink:to="loc_us-gaap_PlanNameAxis_33ec534e-5d37-42e1-9464-bf34e0d9377e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_33ec534e-5d37-42e1-9464-bf34e0d9377e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_33ec534e-5d37-42e1-9464-bf34e0d9377e" xlink:to="loc_us-gaap_PlanNameDomain_33ec534e-5d37-42e1-9464-bf34e0d9377e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_127d7f50-5037-40b5-ac3a-edf7129a8a5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_33ec534e-5d37-42e1-9464-bf34e0d9377e" xlink:to="loc_us-gaap_PlanNameDomain_127d7f50-5037-40b5-ac3a-edf7129a8a5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_StockOptionPlan2010Member_1b922a15-87e4-44eb-b758-058acb823a82" xlink:href="band-20251231.xsd#band_StockOptionPlan2010Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_127d7f50-5037-40b5-ac3a-edf7129a8a5b" xlink:to="loc_band_StockOptionPlan2010Member_1b922a15-87e4-44eb-b758-058acb823a82" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_A2017EquityCompensationPlanMember_f8a0e403-6f7f-4efa-9a57-700cdacaebb4" xlink:href="band-20251231.xsd#band_A2017EquityCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_127d7f50-5037-40b5-ac3a-edf7129a8a5b" xlink:to="loc_band_A2017EquityCompensationPlanMember_f8a0e403-6f7f-4efa-9a57-700cdacaebb4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_ad9c9a8a-e840-4539-82a5-b1b3503fd2c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_ad9c9a8a-e840-4539-82a5-b1b3503fd2c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_311d1b9c-2d7a-49d5-b502-9f65ff1697f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_311d1b9c-2d7a-49d5-b502-9f65ff1697f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_CommonStockCapitalSharesReservedForFutureIssuancePercentIncrease_890f05db-6906-4748-8d12-46588a85e005" xlink:href="band-20251231.xsd#band_CommonStockCapitalSharesReservedForFutureIssuancePercentIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:to="loc_band_CommonStockCapitalSharesReservedForFutureIssuancePercentIncrease_890f05db-6906-4748-8d12-46588a85e005" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_e051eaa2-ec42-4801-a60f-264bf9fc4e0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_e051eaa2-ec42-4801-a60f-264bf9fc4e0c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_057db5be-4e73-466e-b57a-28b4d1621997" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_057db5be-4e73-466e-b57a-28b4d1621997" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_3ce49550-aaef-400d-9c05-8bd319668719" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_3ce49550-aaef-400d-9c05-8bd319668719" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_de716e87-7f52-4cef-a76e-87c95955ee91" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ca2c9b7a-5732-4fa6-ae4c-0100a1fb298f" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_de716e87-7f52-4cef-a76e-87c95955ee91" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensationScheduleofRestrictedStockUnitActivityDetails" xlink:type="simple" xlink:href="band-20251231.xsd#StockBasedCompensationScheduleofRestrictedStockUnitActivityDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensationScheduleofRestrictedStockUnitActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_43e957fa-8886-46f8-bbf5-d233f88d73bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3d3201d9-6640-4cfb-aa25-7492cf932546" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_43e957fa-8886-46f8-bbf5-d233f88d73bb" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3d3201d9-6640-4cfb-aa25-7492cf932546" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_8b1591bc-a50b-4d57-9b09-119f88e1694e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3d3201d9-6640-4cfb-aa25-7492cf932546" xlink:to="loc_us-gaap_AwardTypeAxis_8b1591bc-a50b-4d57-9b09-119f88e1694e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8b1591bc-a50b-4d57-9b09-119f88e1694e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_8b1591bc-a50b-4d57-9b09-119f88e1694e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8b1591bc-a50b-4d57-9b09-119f88e1694e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_64e84ce7-7630-45ff-b4bd-a0518f95912e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_8b1591bc-a50b-4d57-9b09-119f88e1694e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_64e84ce7-7630-45ff-b4bd-a0518f95912e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_2eaae5e1-ff7e-440b-a88e-8542085fb95d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_64e84ce7-7630-45ff-b4bd-a0518f95912e" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_2eaae5e1-ff7e-440b-a88e-8542085fb95d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_da51eb8b-d01d-481c-b61f-40d9b459b7f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_43e957fa-8886-46f8-bbf5-d233f88d73bb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_da51eb8b-d01d-481c-b61f-40d9b459b7f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_0576e413-4b51-4dda-8f91-926b3ce587c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_da51eb8b-d01d-481c-b61f-40d9b459b7f0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_0576e413-4b51-4dda-8f91-926b3ce587c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_a345265e-7af6-430d-83cc-f50553bfff67" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_da51eb8b-d01d-481c-b61f-40d9b459b7f0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_a345265e-7af6-430d-83cc-f50553bfff67" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_6b24cad4-8b8e-4e80-9092-e72c8973781c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_da51eb8b-d01d-481c-b61f-40d9b459b7f0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_6b24cad4-8b8e-4e80-9092-e72c8973781c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_18d993de-5ddc-4433-be3d-d67e840b7c39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_da51eb8b-d01d-481c-b61f-40d9b459b7f0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_18d993de-5ddc-4433-be3d-d67e840b7c39" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_7afa607c-3a51-4b74-b8ba-0e175b537548" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3f7b77cf-3778-4992-8552-f6fb860bb9b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_43e957fa-8886-46f8-bbf5-d233f88d73bb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3f7b77cf-3778-4992-8552-f6fb860bb9b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_66c5ab99-fc7f-4574-87eb-cd1ffbdfb9be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3f7b77cf-3778-4992-8552-f6fb860bb9b4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_66c5ab99-fc7f-4574-87eb-cd1ffbdfb9be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_792722fc-6d17-425e-8233-a42ebd5a14c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3f7b77cf-3778-4992-8552-f6fb860bb9b4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_792722fc-6d17-425e-8233-a42ebd5a14c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_bf4aca5e-1822-4586-b451-84b35b6d3a41" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3f7b77cf-3778-4992-8552-f6fb860bb9b4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_bf4aca5e-1822-4586-b451-84b35b6d3a41" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_020a6191-b8f2-4dd6-bf61-f6998a984069" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3f7b77cf-3778-4992-8552-f6fb860bb9b4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_020a6191-b8f2-4dd6-bf61-f6998a984069" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_27b3fc3b-e5e4-41a6-a431-3659f6be2ca3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_43fe7bcb-b594-4c78-a334-44ae812c762f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3f7b77cf-3778-4992-8552-f6fb860bb9b4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_43fe7bcb-b594-4c78-a334-44ae812c762f" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensationScheduleofTotalStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="band-20251231.xsd#StockBasedCompensationScheduleofTotalStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensationScheduleofTotalStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_0a3d772b-6e8a-46e3-8977-afee3736f938" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_b0788161-2e45-403f-b4d4-b00080c33641" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_0a3d772b-6e8a-46e3-8977-afee3736f938" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_b0788161-2e45-403f-b4d4-b00080c33641" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_bf6bf914-a921-4944-a453-454e73e52d28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_b0788161-2e45-403f-b4d4-b00080c33641" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_bf6bf914-a921-4944-a453-454e73e52d28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_bf6bf914-a921-4944-a453-454e73e52d28_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_bf6bf914-a921-4944-a453-454e73e52d28" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_bf6bf914-a921-4944-a453-454e73e52d28_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_6ac92088-061c-429c-b724-90d12cd1ddde" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_bf6bf914-a921-4944-a453-454e73e52d28" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_6ac92088-061c-429c-b724-90d12cd1ddde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_f92876c0-74f6-4c0d-9598-66a15aa7f0c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6ac92088-061c-429c-b724-90d12cd1ddde" xlink:to="loc_us-gaap_CostOfSalesMember_f92876c0-74f6-4c0d-9598-66a15aa7f0c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_5bcdddff-e506-4042-8b3a-eade3da3b82a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6ac92088-061c-429c-b724-90d12cd1ddde" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_5bcdddff-e506-4042-8b3a-eade3da3b82a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_60c370f9-88b1-4fa0-a28a-58cfde0be92a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6ac92088-061c-429c-b724-90d12cd1ddde" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_60c370f9-88b1-4fa0-a28a-58cfde0be92a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_4bb96991-005c-4e86-907a-ece5c89df27a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6ac92088-061c-429c-b724-90d12cd1ddde" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_4bb96991-005c-4e86-907a-ece5c89df27a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_d8598a1f-441f-4f5e-b74b-31a0ceb4d172" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_0a3d772b-6e8a-46e3-8977-afee3736f938" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_d8598a1f-441f-4f5e-b74b-31a0ceb4d172" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="band-20251231.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_d865993a-7ce0-437b-b095-a3d8445c4365" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_c964d547-a68e-4f51-a1c6-ee95c20d375d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d865993a-7ce0-437b-b095-a3d8445c4365" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_c964d547-a68e-4f51-a1c6-ee95c20d375d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_e6f4a45f-cb3e-4a11-bdee-7f322c2d698d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_c964d547-a68e-4f51-a1c6-ee95c20d375d" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_e6f4a45f-cb3e-4a11-bdee-7f322c2d698d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_e6f4a45f-cb3e-4a11-bdee-7f322c2d698d_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_e6f4a45f-cb3e-4a11-bdee-7f322c2d698d" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_e6f4a45f-cb3e-4a11-bdee-7f322c2d698d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_0892bad2-7fb7-4812-99d6-87a9f567575d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_e6f4a45f-cb3e-4a11-bdee-7f322c2d698d" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_0892bad2-7fb7-4812-99d6-87a9f567575d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_OfficeSpaceMember_0f3b3aef-f204-4d32-8c4d-c066c7ecf62a" xlink:href="band-20251231.xsd#band_OfficeSpaceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_0892bad2-7fb7-4812-99d6-87a9f567575d" xlink:to="loc_band_OfficeSpaceMember_0f3b3aef-f204-4d32-8c4d-c066c7ecf62a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_40d1633c-6eb5-41cd-97d5-e52fac45cc09" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d865993a-7ce0-437b-b095-a3d8445c4365" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_40d1633c-6eb5-41cd-97d5-e52fac45cc09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_fd396cd7-e984-4989-b09b-dccb15b4fad1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d865993a-7ce0-437b-b095-a3d8445c4365" xlink:to="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_fd396cd7-e984-4989-b09b-dccb15b4fad1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary_65c286ac-2a71-455e-bcfc-dd792b2ed8ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d865993a-7ce0-437b-b095-a3d8445c4365" xlink:to="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary_65c286ac-2a71-455e-bcfc-dd792b2ed8ee" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyDamagesSoughtValue_6f05df19-492a-4f9c-9623-07437bee813c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyDamagesSoughtValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d865993a-7ce0-437b-b095-a3d8445c4365" xlink:to="loc_us-gaap_LossContingencyDamagesSoughtValue_6f05df19-492a-4f9c-9623-07437bee813c" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/IncomeTaxesEffectiveIncomeTaxRateReconciliationDetails" xlink:type="simple" xlink:href="band-20251231.xsd#IncomeTaxesEffectiveIncomeTaxRateReconciliationDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IncomeTaxesEffectiveIncomeTaxRateReconciliationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019beef5-1618-7048-a4e6-815d39e2dd23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_019beef5-1618-709a-8012-ac7b922fb8c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019beef5-1618-7048-a4e6-815d39e2dd23" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_019beef5-1618-709a-8012-ac7b922fb8c1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_019beef6-8807-7b30-ac02-076c8858840f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_019beef5-1618-709a-8012-ac7b922fb8c1" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_019beef6-8807-7b30-ac02-076c8858840f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-7b30-ac02-076c8858840f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019beef6-8807-7b30-ac02-076c8858840f" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-7b30-ac02-076c8858840f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-75a8-99c2-870544b095d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019beef6-8807-7b30-ac02-076c8858840f" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-75a8-99c2-870544b095d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_BE_019beefd-3c2e-7a31-bfc4-66834dcc3f27" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_BE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-75a8-99c2-870544b095d9" xlink:to="loc_country_BE_019beefd-3c2e-7a31-bfc4-66834dcc3f27" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_019beefd-7aa2-712a-9fb1-0cbd5cd7006e" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-75a8-99c2-870544b095d9" xlink:to="loc_country_IE_019beefd-7aa2-712a-9fb1-0cbd5cd7006e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_019beef6-8807-7a5f-b1ae-229ff8768483" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-75a8-99c2-870544b095d9" xlink:to="loc_country_GB_019beef6-8807-7a5f-b1ae-229ff8768483" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019beefe-b469-762b-a2dc-57b1cd8c03f2" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-75a8-99c2-870544b095d9" xlink:to="loc_country_US_019beefe-b469-762b-a2dc-57b1cd8c03f2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignTaxJurisdictionOtherMember_019bef0d-d8a0-7341-9313-5f9f7002b36a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignTaxJurisdictionOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019beef6-8807-75a8-99c2-870544b095d9" xlink:to="loc_us-gaap_ForeignTaxJurisdictionOtherMember_019bef0d-d8a0-7341-9313-5f9f7002b36a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019beef5-1618-7048-a4e6-815d39e2dd23" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate_019beef5-1618-79a2-b071-3ea6091b7ef0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate_019beef5-1618-79a2-b071-3ea6091b7ef0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes_019beef6-2e25-7755-992b-68165b62c667" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes_019beef6-2e25-7755-992b-68165b62c667" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential_019beef6-8807-7db4-acc4-2c5a9eaaddd0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential_019beef6-8807-7db4-acc4-2c5a9eaaddd0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_EffectiveIncomeTaxRateReconciliationDeductionInnovationIncomeAmount_019beefa-7188-77a2-a3c6-47bc59d52181" xlink:href="band-20251231.xsd#band_EffectiveIncomeTaxRateReconciliationDeductionInnovationIncomeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_band_EffectiveIncomeTaxRateReconciliationDeductionInnovationIncomeAmount_019beefa-7188-77a2-a3c6-47bc59d52181" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationOtherAdjustments_019beefb-c3dc-7011-8d3c-d81a05585ecf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationOtherAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationOtherAdjustments_019beefb-c3dc-7011-8d3c-d81a05585ecf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationTaxCreditsResearch_019beefe-b469-726f-9e59-5e57470b1ef7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationTaxCreditsResearch"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationTaxCreditsResearch_019beefe-b469-726f-9e59-5e57470b1ef7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationTaxCreditsOther_019beeff-210c-7790-aae9-b980d902206e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationTaxCreditsOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationTaxCreditsOther_019beeff-210c-7790-aae9-b980d902206e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_019beeff-84cb-71dc-8a40-9751eccf26cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_019beeff-84cb-71dc-8a40-9751eccf26cd" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseMealsAndEntertainment_019bef02-6ddf-7b67-8e7c-fcda1fd535b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpenseMealsAndEntertainment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseMealsAndEntertainment_019bef02-6ddf-7b67-8e7c-fcda1fd535b3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost_019bef02-a948-71dc-8093-9a7c675f8097" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost_019bef02-a948-71dc-8093-9a7c675f8097" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseNonDeductibleExecutiveCompensationAmount_019bef08-143e-76c3-bbf0-a07ac8753d9a" xlink:href="band-20251231.xsd#band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseNonDeductibleExecutiveCompensationAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseNonDeductibleExecutiveCompensationAmount_019bef08-143e-76c3-bbf0-a07ac8753d9a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseIncomeFromBranchAmount_019bef04-2b65-786e-9825-1a3306a0d57f" xlink:href="band-20251231.xsd#band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseIncomeFromBranchAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseIncomeFromBranchAmount_019bef04-2b65-786e-9825-1a3306a0d57f" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther_019bef05-d028-7239-b40e-a98f9bab763b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther_019bef05-d028-7239-b40e-a98f9bab763b" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationTaxContingencies_019bef06-2bf4-7799-9e52-d69465ddd392" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationTaxContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxReconciliationTaxContingencies_019bef06-2bf4-7799-9e52-d69465ddd392" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019bef06-e0d8-7875-b40a-55adddffa968" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_019bef0a-5a48-7e29-bfd1-f357d9fabf61" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019bef06-e0d8-7875-b40a-55adddffa968" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019beef5-1618-7048-a4e6-815d39e2dd23" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_019beef5-1618-7f2b-8e6a-a63bfdc8d202" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_019beef5-1618-7f2b-8e6a-a63bfdc8d202" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_019beef6-441f-740c-80be-2691a62d1efb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_019beef6-441f-740c-80be-2691a62d1efb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential_019beef6-8807-7c85-8c8e-32b4831deb0a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential_019beef6-8807-7c85-8c8e-32b4831deb0a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_EffectiveIncomeTaxRateReconciliationDeductionInnovationIncomePercent_019beefa-0b45-703d-8a68-5dd132a9f474" xlink:href="band-20251231.xsd#band_EffectiveIncomeTaxRateReconciliationDeductionInnovationIncomePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_band_EffectiveIncomeTaxRateReconciliationDeductionInnovationIncomePercent_019beefa-0b45-703d-8a68-5dd132a9f474" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments_019beefb-f76d-72cd-b965-a97550aab1dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments_019beefb-f76d-72cd-b965-a97550aab1dc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch_019beefe-e94a-7d9b-b65e-3990cdd45d1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch_019beefe-e94a-7d9b-b65e-3990cdd45d1a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther_019beeff-4ef6-7bdd-bd7a-05621c89c172" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther_019beeff-4ef6-7bdd-bd7a-05621c89c172" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance_019beeff-9fc3-7afd-8570-a374b27e07a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance_019beeff-9fc3-7afd-8570-a374b27e07a7" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment_019bef09-6eb5-77a8-b9d4-7366dbfeea2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment_019bef09-6eb5-77a8-b9d4-7366dbfeea2b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost_019bef09-a96c-7207-93d8-6ac7aabcbde2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost_019bef09-a96c-7207-93d8-6ac7aabcbde2" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseNonDeductibleExecutiveCompensationPercent_019bef03-8ff6-78e4-930c-c9f914f4c9c9" xlink:href="band-20251231.xsd#band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseNonDeductibleExecutiveCompensationPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseNonDeductibleExecutiveCompensationPercent_019bef03-8ff6-78e4-930c-c9f914f4c9c9" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseIncomeFromBranchPercent_019bef08-9338-7e77-89ad-56cffc81ced2" xlink:href="band-20251231.xsd#band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseIncomeFromBranchPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_band_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseIncomeFromBranchPercent_019bef08-9338-7e77-89ad-56cffc81ced2" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther_019bef05-fa51-729d-b6af-80364388664e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther_019bef05-fa51-729d-b6af-80364388664e" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxContingencies_019bef06-5855-7b4e-8894-572fe1d1fb72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTaxContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxContingencies_019bef06-5855-7b4e-8894-572fe1d1fb72" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019bef07-0711-71a0-9a56-5a5fb7cbdd06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_019bef0f-7feb-7b69-ac88-93d5d7b81788" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019bef07-0711-71a0-9a56-5a5fb7cbdd06" xlink:type="arc" order="15"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/IncomeTaxesAdditionalInformationDetails" xlink:type="simple" xlink:href="band-20251231.xsd#IncomeTaxesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IncomeTaxesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_202239a7-5888-47c0-8c07-44f0877fadc9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwardsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_202239a7-5888-47c0-8c07-44f0877fadc9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_82ce6843-703f-4ead-b4de-d1653e74b541" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_202239a7-5888-47c0-8c07-44f0877fadc9" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_82ce6843-703f-4ead-b4de-d1653e74b541" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_82ce6843-703f-4ead-b4de-d1653e74b541_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_82ce6843-703f-4ead-b4de-d1653e74b541" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_82ce6843-703f-4ead-b4de-d1653e74b541_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_90a3b8f0-1473-42db-b13a-064b3633e6b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_82ce6843-703f-4ead-b4de-d1653e74b541" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_90a3b8f0-1473-42db-b13a-064b3633e6b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalMinistryOfFinanceGermanyMember_3a57f211-1500-417d-a150-db0b6d4ec851" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalMinistryOfFinanceGermanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_90a3b8f0-1473-42db-b13a-064b3633e6b6" xlink:to="loc_us-gaap_FederalMinistryOfFinanceGermanyMember_3a57f211-1500-417d-a150-db0b6d4ec851" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxAndCustomsAdministrationNetherlandsMember_01c2c4f4-3dc7-4351-a358-3a03e7792b68" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxAndCustomsAdministrationNetherlandsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_90a3b8f0-1473-42db-b13a-064b3633e6b6" xlink:to="loc_us-gaap_TaxAndCustomsAdministrationNetherlandsMember_01c2c4f4-3dc7-4351-a358-3a03e7792b68" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_c5ce3cd7-eeba-4a0b-a4d2-09bdef6a5b6c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_90a3b8f0-1473-42db-b13a-064b3633e6b6" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_c5ce3cd7-eeba-4a0b-a4d2-09bdef6a5b6c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_1df2eb6a-6102-45ed-a256-15d6745fcd56" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_1df2eb6a-6102-45ed-a256-15d6745fcd56" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossAttributableToParent_c7c7cd4c-a4bf-4c9f-8b0e-55b2c63d6221" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_IncomeLossAttributableToParent_c7c7cd4c-a4bf-4c9f-8b0e-55b2c63d6221" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_0e20c1f6-f30a-4fbe-9c7b-5ed6aee23ac9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_0e20c1f6-f30a-4fbe-9c7b-5ed6aee23ac9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_019bef15-8a2a-7721-8a9c-6744c70e6708" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_019bef15-8a2a-7721-8a9c-6744c70e6708" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_1ed0381f-cec7-434d-82f4-1622614ba623" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_OperatingLossCarryforwards_1ed0381f-cec7-434d-82f4-1622614ba623" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAmount_a1213e49-8f78-49a2-8b47-ff53c575ad4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_TaxCreditCarryforwardAmount_a1213e49-8f78-49a2-8b47-ff53c575ad4d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_f2fa7909-d611-4eac-b158-5972f06ae53c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_UnrecognizedTaxBenefits_f2fa7909-d611-4eac-b158-5972f06ae53c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense_3433c073-e8d7-4a8c-946a-ed12005e0fbc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_ec2c8329-c701-4474-bda5-2735a48a97e4" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense_3433c073-e8d7-4a8c-946a-ed12005e0fbc" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/IncomeTaxesIncomeTaxesPaidRefundedDetails" xlink:type="simple" xlink:href="band-20251231.xsd#IncomeTaxesIncomeTaxesPaidRefundedDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IncomeTaxesIncomeTaxesPaidRefundedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019bef17-e8d5-7c85-9ea9-0ef35f09bb43" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionTable_019bef17-e8d5-7f86-8315-43b665338d04" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidByIndividualJurisdictionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019bef17-e8d5-7c85-9ea9-0ef35f09bb43" xlink:to="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionTable_019bef17-e8d5-7f86-8315-43b665338d04" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_019bef17-e8d5-7612-a741-41395e168aaa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionTable_019bef17-e8d5-7f86-8315-43b665338d04" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_019bef17-e8d5-7612-a741-41395e168aaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7612-a741-41395e168aaa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019bef17-e8d5-7612-a741-41395e168aaa" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7612-a741-41395e168aaa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019bef17-e8d5-7612-a741-41395e168aaa" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_CA_019bef1a-14d2-74d2-b545-6824cd902baf" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_stpr_CA_019bef1a-14d2-74d2-b545-6824cd902baf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_DC_019c227b-ffe4-73f6-8b5a-e5920fbfd18a" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_DC"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_stpr_DC_019c227b-ffe4-73f6-8b5a-e5920fbfd18a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_DE_019bef1a-b742-795e-a208-698f71061b5a" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_DE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_stpr_DE_019bef1a-b742-795e-a208-698f71061b5a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_NJ_019bef1b-10c9-7ab2-a11b-c543b45f3b8c" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_NJ"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_stpr_NJ_019bef1b-10c9-7ab2-a11b-c543b45f3b8c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_PA_019bef1b-6075-74df-b5b9-df1b86456291" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_PA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_stpr_PA_019bef1b-6075-74df-b5b9-df1b86456291" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalTaxJurisdictionOtherMember_019bef1b-a03b-7cad-a7b9-f504e5aa1c58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalTaxJurisdictionOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_us-gaap_StateAndLocalTaxJurisdictionOtherMember_019bef1b-a03b-7cad-a7b9-f504e5aa1c58" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_BE_019bef1c-41f2-73d3-8cda-5bbcceb79f43" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_BE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_country_BE_019bef1c-41f2-73d3-8cda-5bbcceb79f43" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_019bef1c-81a5-772d-b755-4f1f1c0e0b43" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_country_IE_019bef1c-81a5-772d-b755-4f1f1c0e0b43" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignTaxJurisdictionOtherMember_019bef17-e8d5-77d9-b4a5-e6cb28c527da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignTaxJurisdictionOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019bef17-e8d5-7312-b21f-190b4c991162" xlink:to="loc_us-gaap_ForeignTaxJurisdictionOtherMember_019bef17-e8d5-77d9-b4a5-e6cb28c527da" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_019bef17-e8d5-714d-a2a0-e0916a0d7269" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidFederalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019bef17-e8d5-7c85-9ea9-0ef35f09bb43" xlink:to="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_019bef17-e8d5-714d-a2a0-e0916a0d7269" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_019bef17-e8d5-7ecb-a76b-5c8634656d82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019bef17-e8d5-7c85-9ea9-0ef35f09bb43" xlink:to="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_019bef17-e8d5-7ecb-a76b-5c8634656d82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidForeignAfterRefundReceivedAbstract_019bef17-e8d5-782c-95b4-e11c967032fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidForeignAfterRefundReceivedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019bef17-e8d5-7c85-9ea9-0ef35f09bb43" xlink:to="loc_us-gaap_IncomeTaxPaidForeignAfterRefundReceivedAbstract_019bef17-e8d5-782c-95b4-e11c967032fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidForeignAfterRefundReceived_019bef17-e8d5-7f27-b747-8bf4849a6224" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidForeignAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidForeignAfterRefundReceivedAbstract_019bef17-e8d5-782c-95b4-e11c967032fd" xlink:to="loc_us-gaap_IncomeTaxPaidForeignAfterRefundReceived_019bef17-e8d5-7f27-b747-8bf4849a6224" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_019bef17-e8d5-7914-91d0-7b06cda99c44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019bef17-e8d5-7c85-9ea9-0ef35f09bb43" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_019bef17-e8d5-7914-91d0-7b06cda99c44" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/BasicandDilutedLossperCommonShareScheduleofComponentsofBasicandDilutedLossIncomePerShareDetails" xlink:type="simple" xlink:href="band-20251231.xsd#BasicandDilutedLossperCommonShareScheduleofComponentsofBasicandDilutedLossIncomePerShareDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/BasicandDilutedLossperCommonShareScheduleofComponentsofBasicandDilutedLossIncomePerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_bc3b928c-0b2e-4311-afdb-9f8b9fd0fd90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_56c2b7d9-3602-4ae0-b169-5876835c3015" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_bc3b928c-0b2e-4311-afdb-9f8b9fd0fd90" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_56c2b7d9-3602-4ae0-b169-5876835c3015" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_af18d20e-0f04-47b8-9a5f-f3192808e172" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_56c2b7d9-3602-4ae0-b169-5876835c3015" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_af18d20e-0f04-47b8-9a5f-f3192808e172" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_af18d20e-0f04-47b8-9a5f-f3192808e172_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_af18d20e-0f04-47b8-9a5f-f3192808e172" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_af18d20e-0f04-47b8-9a5f-f3192808e172_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_53ca980a-287b-489d-acba-492784665bd3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_af18d20e-0f04-47b8-9a5f-f3192808e172" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_53ca980a-287b-489d-acba-492784665bd3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_0a471662-2d55-4f6e-9cc3-20d8cca85c11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_53ca980a-287b-489d-acba-492784665bd3" xlink:to="loc_us-gaap_EmployeeStockOptionMember_0a471662-2d55-4f6e-9cc3-20d8cca85c11" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_30cd09e8-038a-446d-9478-1cba5e016471" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_53ca980a-287b-489d-acba-492784665bd3" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_30cd09e8-038a-446d-9478-1cba5e016471" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_c74f84af-2e1a-4656-84f1-6b29a06a8979" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_bc3b928c-0b2e-4311-afdb-9f8b9fd0fd90" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_c74f84af-2e1a-4656-84f1-6b29a06a8979" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_EarningsPerShareBasicAndDilutedEPSAbstract_2b144730-81f5-465b-a211-3659c63ba87e" xlink:href="band-20251231.xsd#band_EarningsPerShareBasicAndDilutedEPSAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_bc3b928c-0b2e-4311-afdb-9f8b9fd0fd90" xlink:to="loc_band_EarningsPerShareBasicAndDilutedEPSAbstract_2b144730-81f5-465b-a211-3659c63ba87e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_acdd33c0-c11b-4248-876a-34e0bd09864b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_band_EarningsPerShareBasicAndDilutedEPSAbstract_2b144730-81f5-465b-a211-3659c63ba87e" xlink:to="loc_us-gaap_EarningsPerShareBasic_acdd33c0-c11b-4248-876a-34e0bd09864b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_0fd2e474-3f52-4907-a1dd-25e4ba32df52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_band_EarningsPerShareBasicAndDilutedEPSAbstract_2b144730-81f5-465b-a211-3659c63ba87e" xlink:to="loc_us-gaap_EarningsPerShareDiluted_0fd2e474-3f52-4907-a1dd-25e4ba32df52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_496e9805-a8dd-47bf-8f31-6883028cbd6f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_bc3b928c-0b2e-4311-afdb-9f8b9fd0fd90" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_496e9805-a8dd-47bf-8f31-6883028cbd6f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_1b117253-6b5d-458f-8751-1d231bd4f91c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_496e9805-a8dd-47bf-8f31-6883028cbd6f" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_1b117253-6b5d-458f-8751-1d231bd4f91c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_5988d914-bff7-439b-81d1-dfda25eedfec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_496e9805-a8dd-47bf-8f31-6883028cbd6f" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_5988d914-bff7-439b-81d1-dfda25eedfec" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/BasicandDilutedLossperCommonShareScheduleofAntidilutiveCommonSharesExcludedfromCalculationofDilutedNetLossIncomePerCommonShareDetails" xlink:type="simple" xlink:href="band-20251231.xsd#BasicandDilutedLossperCommonShareScheduleofAntidilutiveCommonSharesExcludedfromCalculationofDilutedNetLossIncomePerCommonShareDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/BasicandDilutedLossperCommonShareScheduleofAntidilutiveCommonSharesExcludedfromCalculationofDilutedNetLossIncomePerCommonShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d60d8d43-ce25-42fa-9b35-931b5df15b3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4c32bc40-97bc-4bc2-9057-eb8a5e693fe5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d60d8d43-ce25-42fa-9b35-931b5df15b3b" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4c32bc40-97bc-4bc2-9057-eb8a5e693fe5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a7dd4819-14d7-47fe-a7cd-988001520e21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4c32bc40-97bc-4bc2-9057-eb8a5e693fe5" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a7dd4819-14d7-47fe-a7cd-988001520e21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a7dd4819-14d7-47fe-a7cd-988001520e21_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a7dd4819-14d7-47fe-a7cd-988001520e21" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a7dd4819-14d7-47fe-a7cd-988001520e21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_df2b6770-a391-4a3e-aab8-df577da38663" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a7dd4819-14d7-47fe-a7cd-988001520e21" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_df2b6770-a391-4a3e-aab8-df577da38663" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_f035e0ea-0b86-4c6b-b841-fca849357b46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_df2b6770-a391-4a3e-aab8-df577da38663" xlink:to="loc_us-gaap_EmployeeStockOptionMember_f035e0ea-0b86-4c6b-b841-fca849357b46" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_a7fd0dce-d355-4fb3-abae-e5bafc18ffde" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_df2b6770-a391-4a3e-aab8-df577da38663" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_a7fd0dce-d355-4fb3-abae-e5bafc18ffde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_3dfe0f04-59fe-4870-95e5-7a3b9c0bee31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_df2b6770-a391-4a3e-aab8-df577da38663" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_3dfe0f04-59fe-4870-95e5-7a3b9c0bee31" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_c3c2748c-7ee2-4809-baa6-d8e155f11282" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d60d8d43-ce25-42fa-9b35-931b5df15b3b" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_c3c2748c-7ee2-4809-baa6-d8e155f11282" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.bandwidth.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="band-20251231.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_0d0d6746-bced-46be-89c6-b745c64f60af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_c0cd7a5f-6d7c-49c2-a95c-a1ff37dc72d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_0d0d6746-bced-46be-89c6-b745c64f60af" xlink:to="loc_us-gaap_SubsequentEventTable_c0cd7a5f-6d7c-49c2-a95c-a1ff37dc72d3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_716e532a-e293-43a5-b2ef-3d1fd8ec8157" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_c0cd7a5f-6d7c-49c2-a95c-a1ff37dc72d3" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_716e532a-e293-43a5-b2ef-3d1fd8ec8157" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_716e532a-e293-43a5-b2ef-3d1fd8ec8157_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_716e532a-e293-43a5-b2ef-3d1fd8ec8157" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_716e532a-e293-43a5-b2ef-3d1fd8ec8157_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_76032f7c-7151-4a11-a9c4-0bd00c6e2ad0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_716e532a-e293-43a5-b2ef-3d1fd8ec8157" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_76032f7c-7151-4a11-a9c4-0bd00c6e2ad0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_c5a0f586-e098-48a1-b4e7-42adbb9bfba7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_76032f7c-7151-4a11-a9c4-0bd00c6e2ad0" xlink:to="loc_us-gaap_SubsequentEventMember_c5a0f586-e098-48a1-b4e7-42adbb9bfba7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019c513f-9687-7f7c-a295-89383834d093" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_0d0d6746-bced-46be-89c6-b745c64f60af" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019c513f-9687-7f7c-a295-89383834d093" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_DarylE.RaifordMember_019b00b0-a48b-7f12-8910-5f1b8f571214" xlink:href="band-20251231.xsd#band_DarylE.RaifordMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_band_DarylE.RaifordMember_019b00b0-a48b-7f12-8910-5f1b8f571214" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_band_KadeRossMember_019b00b1-8d53-7051-b36e-bb6308bfaacb" xlink:href="band-20251231.xsd#band_KadeRossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_band_KadeRossMember_019b00b1-8d53-7051-b36e-bb6308bfaacb" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
