<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<link:linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <link:roleRef roleURI="http://www.bandwidth.com/role/BasicAndDilutedIncomeLossPerCommonShare" xlink:href="band-20170930.xsd#BasicAndDilutedIncomeLossPerCommonShare" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/BasicAndDilutedIncomeLossPerCommonShareAntidilutiveSecuritiesExcludedFromComputationDetails" xlink:href="band-20170930.xsd#BasicAndDilutedIncomeLossPerCommonShareAntidilutiveSecuritiesExcludedFromComputationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/BasicAndDilutedIncomeLossPerCommonShareComponentsOfBasicAndDilutedEarningsPerShareDetails" xlink:href="band-20170930.xsd#BasicAndDilutedIncomeLossPerCommonShareComponentsOfBasicAndDilutedEarningsPerShareDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/BasicAndDilutedIncomeLossPerCommonShareTables" xlink:href="band-20170930.xsd#BasicAndDilutedIncomeLossPerCommonShareTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CommitmentsAndContingencies" xlink:href="band-20170930.xsd#CommitmentsAndContingencies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CommitmentsAndContingenciesContractualObligationsDetails" xlink:href="band-20170930.xsd#CommitmentsAndContingenciesContractualObligationsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CommitmentsAndContingenciesFutureMinimumLeaseReceiptsDetails" xlink:href="band-20170930.xsd#CommitmentsAndContingenciesFutureMinimumLeaseReceiptsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CommitmentsAndContingenciesFutureMinimumRentalPaymentsForOperatingLeasesDetails" xlink:href="band-20170930.xsd#CommitmentsAndContingenciesFutureMinimumRentalPaymentsForOperatingLeasesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CommitmentsAndContingenciesOperatingLeasesDetails" xlink:href="band-20170930.xsd#CommitmentsAndContingenciesOperatingLeasesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CommitmentsAndContingenciesTables" xlink:href="band-20170930.xsd#CommitmentsAndContingenciesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CondensedConsolidatedBalanceSheets" xlink:href="band-20170930.xsd#CondensedConsolidatedBalanceSheets" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CondensedConsolidatedStatementsOfCashFlows" xlink:href="band-20170930.xsd#CondensedConsolidatedStatementsOfCashFlows" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/CondensedConsolidatedStatementsOfOperationsAndComprehensiveIncome" xlink:href="band-20170930.xsd#CondensedConsolidatedStatementsOfOperationsAndComprehensiveIncome" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/Debt" xlink:href="band-20170930.xsd#Debt" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtCapitalLeasesDetails" xlink:href="band-20170930.xsd#DebtCapitalLeasesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtFuturePaymentsUnderTermLoanDetails" xlink:href="band-20170930.xsd#DebtFuturePaymentsUnderTermLoanDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtNarrativeDetails" xlink:href="band-20170930.xsd#DebtNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtRemainingPaymentsDueOnCapitalLeaseObligationsDetails" xlink:href="band-20170930.xsd#DebtRemainingPaymentsDueOnCapitalLeaseObligationsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DebtTables" xlink:href="band-20170930.xsd#DebtTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DiscontinuedOperations" xlink:href="band-20170930.xsd#DiscontinuedOperations" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DiscontinuedOperationsDetails" xlink:href="band-20170930.xsd#DiscontinuedOperationsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DiscontinuedOperationsTables" xlink:href="band-20170930.xsd#DiscontinuedOperationsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/DocumentAndEntityInformationDocumentAndEntityInformation" xlink:href="band-20170930.xsd#DocumentAndEntityInformationDocumentAndEntityInformation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/EmployeeBenefitPlan" xlink:href="band-20170930.xsd#EmployeeBenefitPlan" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/EmployeeBenefitPlanDetails" xlink:href="band-20170930.xsd#EmployeeBenefitPlanDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/FairValueOfFinancialInstruments" xlink:href="band-20170930.xsd#FairValueOfFinancialInstruments" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/FairValueOfFinancialInstrumentsDetails" xlink:href="band-20170930.xsd#FairValueOfFinancialInstrumentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/FairValueOfFinancialInstrumentsTables" xlink:href="band-20170930.xsd#FairValueOfFinancialInstrumentsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/FinancialStatementComponents" xlink:href="band-20170930.xsd#FinancialStatementComponents" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/FinancialStatementComponentsAccountsReceivableNetOfAllowanceForDoubtfulAccountsDetails" xlink:href="band-20170930.xsd#FinancialStatementComponentsAccountsReceivableNetOfAllowanceForDoubtfulAccountsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/FinancialStatementComponentsAccruedExpensesAndOtherCurrentLiabilitiesDetails" xlink:href="band-20170930.xsd#FinancialStatementComponentsAccruedExpensesAndOtherCurrentLiabilitiesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/FinancialStatementComponentsAllowanceForDoubtfulAccountsDetails" xlink:href="band-20170930.xsd#FinancialStatementComponentsAllowanceForDoubtfulAccountsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/FinancialStatementComponentsTables" xlink:href="band-20170930.xsd#FinancialStatementComponentsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/IncomeTaxes" xlink:href="band-20170930.xsd#IncomeTaxes" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/IncomeTaxesDetails" xlink:href="band-20170930.xsd#IncomeTaxesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/IntangibleAssets" xlink:href="band-20170930.xsd#IntangibleAssets" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/IntangibleAssetsAmortizationExpenseDetails" xlink:href="band-20170930.xsd#IntangibleAssetsAmortizationExpenseDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/IntangibleAssetsFutureEstimatedAmortizationExpenseDetails" xlink:href="band-20170930.xsd#IntangibleAssetsFutureEstimatedAmortizationExpenseDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/IntangibleAssetsSummaryOfIntangibleAssetsDetails" xlink:href="band-20170930.xsd#IntangibleAssetsSummaryOfIntangibleAssetsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/IntangibleAssetsTables" xlink:href="band-20170930.xsd#IntangibleAssetsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/OrganizationAndDescriptionOfBusiness" xlink:href="band-20170930.xsd#OrganizationAndDescriptionOfBusiness" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/OrganizationAndDescriptionOfBusinessDetails" xlink:href="band-20170930.xsd#OrganizationAndDescriptionOfBusinessDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/PropertyAndEquipment" xlink:href="band-20170930.xsd#PropertyAndEquipment" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/PropertyAndEquipmentDepreciationExpenseDetails" xlink:href="band-20170930.xsd#PropertyAndEquipmentDepreciationExpenseDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/PropertyAndEquipmentSoftwareDevelopmentCostsDetails" xlink:href="band-20170930.xsd#PropertyAndEquipmentSoftwareDevelopmentCostsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/PropertyAndEquipmentSummaryOfPropertyAndEquipmentNetDetails" xlink:href="band-20170930.xsd#PropertyAndEquipmentSummaryOfPropertyAndEquipmentNetDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/PropertyAndEquipmentTables" xlink:href="band-20170930.xsd#PropertyAndEquipmentTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/RedeemableConvertiblePreferredStockAndStockholdersDeficitDetails" xlink:href="band-20170930.xsd#RedeemableConvertiblePreferredStockAndStockholdersDeficitDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/RedeemableConvertiblePreferredStockAndStockholdersDeficitRedeemableConvertiblePreferredStockAndStockholdersDeficitNotes" xlink:href="band-20170930.xsd#RedeemableConvertiblePreferredStockAndStockholdersDeficitRedeemableConvertiblePreferredStockAndStockholdersDeficitNotes" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/RelatedParties" xlink:href="band-20170930.xsd#RelatedParties" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/RelatedPartiesDetails" xlink:href="band-20170930.xsd#RelatedPartiesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/SegmentAndGeographicInformation" xlink:href="band-20170930.xsd#SegmentAndGeographicInformation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/SegmentAndGeographicInformationReconciliationOfRevenueByGeographicAreaDetails" xlink:href="band-20170930.xsd#SegmentAndGeographicInformationReconciliationOfRevenueByGeographicAreaDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/SegmentAndGeographicInformationReconciliationOfSegmentProfitLossDetails" xlink:href="band-20170930.xsd#SegmentAndGeographicInformationReconciliationOfSegmentProfitLossDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/SegmentAndGeographicInformationTables" xlink:href="band-20170930.xsd#SegmentAndGeographicInformationTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensation" xlink:href="band-20170930.xsd#StockBasedCompensation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensationAllocatedStockBasedCompensationExpenseDetails" xlink:href="band-20170930.xsd#StockBasedCompensationAllocatedStockBasedCompensationExpenseDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensationDetails" xlink:href="band-20170930.xsd#StockBasedCompensationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensationFairValueOfOptionsGrantedUsingBlackScholesPricingModelDetails" xlink:href="band-20170930.xsd#StockBasedCompensationFairValueOfOptionsGrantedUsingBlackScholesPricingModelDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensationStockOptionActivityDetails" xlink:href="band-20170930.xsd#StockBasedCompensationStockOptionActivityDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/StockBasedCompensationTables" xlink:href="band-20170930.xsd#StockBasedCompensationTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/SubsequentEvents" xlink:href="band-20170930.xsd#SubsequentEvents" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/SubsequentEventsDetails" xlink:href="band-20170930.xsd#SubsequentEventsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/SummaryOfSignificantAccountingPolicies" xlink:href="band-20170930.xsd#SummaryOfSignificantAccountingPolicies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.bandwidth.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:href="band-20170930.xsd#SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/BasicAndDilutedIncomeLossPerCommonShare" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/BasicAndDilutedIncomeLossPerCommonShareAntidilutiveSecuritiesExcludedFromComputationDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1010178183E50C44EC81513E59722D25" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_C3B4E02D468240467A02513E597184A6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1010178183E50C44EC81513E59722D25" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_C3B4E02D468240467A02513E597184A6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_BAC1A8A23DB441E89816513E59726D0C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_C3B4E02D468240467A02513E597184A6" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_BAC1A8A23DB441E89816513E59726D0C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_032243B5AACD3F33DE34513E5972E88A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_BAC1A8A23DB441E89816513E59726D0C" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_032243B5AACD3F33DE34513E5972E88A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_032243B5AACD3F33DE34513E5972E88A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_BAC1A8A23DB441E89816513E59726D0C" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_032243B5AACD3F33DE34513E5972E88A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RedeemableConvertiblePreferredStockMember" xlink:label="loc_us-gaap_RedeemableConvertiblePreferredStockMember_B1DF0A4160078CB8AE49513E59722C85" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_032243B5AACD3F33DE34513E5972E88A" xlink:to="loc_us-gaap_RedeemableConvertiblePreferredStockMember_B1DF0A4160078CB8AE49513E59722C85" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_02F5EBE46E2C5C76361D513E5972C26C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_032243B5AACD3F33DE34513E5972E88A" xlink:to="loc_us-gaap_EmployeeStockOptionMember_02F5EBE46E2C5C76361D513E5972C26C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_F734458AC207F1EC45F2513E59728260" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1010178183E50C44EC81513E59722D25" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_F734458AC207F1EC45F2513E59728260" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/BasicAndDilutedIncomeLossPerCommonShareComponentsOfBasicAndDilutedEarningsPerShareDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/BasicAndDilutedIncomeLossPerCommonShareTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CommitmentsAndContingencies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CommitmentsAndContingenciesContractualObligationsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherCommitmentsLineItems" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_477ADA176D95E650508A513E5992A57C" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherCommitmentsTable" xlink:label="loc_us-gaap_OtherCommitmentsTable_27F6A5BA8DB50A7A3DFE513E5992E71F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_477ADA176D95E650508A513E5992A57C" xlink:to="loc_us-gaap_OtherCommitmentsTable_27F6A5BA8DB50A7A3DFE513E5992E71F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis_4CC2F4D224734A3B5800513E59926734" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_27F6A5BA8DB50A7A3DFE513E5992E71F" xlink:to="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis_4CC2F4D224734A3B5800513E59926734" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain" xlink:label="loc_us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain_3E1A24F9A0AFC6EBFB33513E5992EB17_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis_4CC2F4D224734A3B5800513E59926734" xlink:to="loc_us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain_3E1A24F9A0AFC6EBFB33513E5992EB17_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain" xlink:label="loc_us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain_3E1A24F9A0AFC6EBFB33513E5992EB17" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis_4CC2F4D224734A3B5800513E59926734" xlink:to="loc_us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain_3E1A24F9A0AFC6EBFB33513E5992EB17" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CapitalAdditionsMember" xlink:label="loc_us-gaap_CapitalAdditionsMember_817C359CBAA41744A8E4513E5992EFC5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_UnconditionalPurchaseObligationCategoryOfGoodsOrServicesAcquiredDomain_3E1A24F9A0AFC6EBFB33513E5992EB17" xlink:to="loc_us-gaap_CapitalAdditionsMember_817C359CBAA41744A8E4513E5992EFC5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherCommitmentsAxis" xlink:label="loc_us-gaap_OtherCommitmentsAxis_5A2D4BFEBB9ADFE23DA8513E5992DE5B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_27F6A5BA8DB50A7A3DFE513E5992E71F" xlink:to="loc_us-gaap_OtherCommitmentsAxis_5A2D4BFEBB9ADFE23DA8513E5992DE5B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="loc_us-gaap_OtherCommitmentsDomain_B08B051D088371D93CE6513E5992E939_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OtherCommitmentsAxis_5A2D4BFEBB9ADFE23DA8513E5992DE5B" xlink:to="loc_us-gaap_OtherCommitmentsDomain_B08B051D088371D93CE6513E5992E939_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="loc_us-gaap_OtherCommitmentsDomain_B08B051D088371D93CE6513E5992E939" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OtherCommitmentsAxis_5A2D4BFEBB9ADFE23DA8513E5992DE5B" xlink:to="loc_us-gaap_OtherCommitmentsDomain_B08B051D088371D93CE6513E5992E939" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_OtherCommitmentsFutureMinimumAnnualPayments" xlink:label="loc_band_OtherCommitmentsFutureMinimumAnnualPayments_577D3AFDFBFCB0E1B6AC513E5992CA7D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_477ADA176D95E650508A513E5992A57C" xlink:to="loc_band_OtherCommitmentsFutureMinimumAnnualPayments_577D3AFDFBFCB0E1B6AC513E5992CA7D" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_OtherCommitmentTermofAgreement" xlink:label="loc_band_OtherCommitmentTermofAgreement_968EEB4441D1D4132318513E59924EF6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_477ADA176D95E650508A513E5992A57C" xlink:to="loc_band_OtherCommitmentTermofAgreement_968EEB4441D1D4132318513E59924EF6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_712AF246079671CE6310513E599233ED" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_477ADA176D95E650508A513E5992A57C" xlink:to="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_712AF246079671CE6310513E599233ED" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CommitmentsAndContingenciesFutureMinimumLeaseReceiptsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CommitmentsAndContingenciesFutureMinimumRentalPaymentsForOperatingLeasesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CommitmentsAndContingenciesOperatingLeasesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LesseeLeaseDescriptionLineItems" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_3723759B1AFC0E6DB576516F08545F74" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LesseeLeaseDescriptionTable" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_6942C9FC83DEEA8947FB516F0853028F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_3723759B1AFC0E6DB576516F08545F74" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_6942C9FC83DEEA8947FB516F0853028F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseArrangementTypeAxis" xlink:label="loc_us-gaap_LeaseArrangementTypeAxis_C54D5E3605E0DDD50700516F08539F35" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_6942C9FC83DEEA8947FB516F0853028F" xlink:to="loc_us-gaap_LeaseArrangementTypeAxis_C54D5E3605E0DDD50700516F08539F35" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:label="loc_us-gaap_LeaseArrangementTypeDomain_EEDC536B5C02ED35CC66516F08539663_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LeaseArrangementTypeAxis_C54D5E3605E0DDD50700516F08539F35" xlink:to="loc_us-gaap_LeaseArrangementTypeDomain_EEDC536B5C02ED35CC66516F08539663_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:label="loc_us-gaap_LeaseArrangementTypeDomain_EEDC536B5C02ED35CC66516F08539663" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LeaseArrangementTypeAxis_C54D5E3605E0DDD50700516F08539F35" xlink:to="loc_us-gaap_LeaseArrangementTypeDomain_EEDC536B5C02ED35CC66516F08539663" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_AmendedOperatingLeaseAgreementOriginalMember" xlink:label="loc_band_AmendedOperatingLeaseAgreementOriginalMember_7CA9542D7A225D23E041516F0853AD7C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseArrangementTypeDomain_EEDC536B5C02ED35CC66516F08539663" xlink:to="loc_band_AmendedOperatingLeaseAgreementOriginalMember_7CA9542D7A225D23E041516F0853AD7C" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_AmendedOperatingLeaseAgreementExtensionMember" xlink:label="loc_band_AmendedOperatingLeaseAgreementExtensionMember_463725F675D0C1D37174516F08544124" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseArrangementTypeDomain_EEDC536B5C02ED35CC66516F08539663" xlink:to="loc_band_AmendedOperatingLeaseAgreementExtensionMember_463725F675D0C1D37174516F08544124" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_6C44F380F996C6D5A865516F085449AA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_6942C9FC83DEEA8947FB516F0853028F" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_6C44F380F996C6D5A865516F085449AA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_E579B126A9967E9989BF516F08542616_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_6C44F380F996C6D5A865516F085449AA" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_E579B126A9967E9989BF516F08542616_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_E579B126A9967E9989BF516F08542616" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_6C44F380F996C6D5A865516F085449AA" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_E579B126A9967E9989BF516F08542616" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_E7A25D6E17EDABFB68CD516F08540D38" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_E579B126A9967E9989BF516F08542616" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_E7A25D6E17EDABFB68CD516F08540D38" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract" xlink:label="loc_us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract_6EC4112B8398E5E9D915516F08546852" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_3723759B1AFC0E6DB576516F08545F74" xlink:to="loc_us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract_6EC4112B8398E5E9D915516F08546852" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AreaOfRealEstateProperty" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_2452F1E33F350409BCEF516F0854C2B7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_3723759B1AFC0E6DB576516F08545F74" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_2452F1E33F350409BCEF516F0854C2B7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseAndRentalExpense" xlink:label="loc_us-gaap_LeaseAndRentalExpense_467C20DC9689E5527AC1516F085417D6" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_3723759B1AFC0E6DB576516F08545F74" xlink:to="loc_us-gaap_LeaseAndRentalExpense_467C20DC9689E5527AC1516F085417D6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue" xlink:label="loc_us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue_B80E007DE633D0BD2715516F0854AB49" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_3723759B1AFC0E6DB576516F08545F74" xlink:to="loc_us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue_B80E007DE633D0BD2715516F0854AB49" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CommitmentsAndContingenciesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CondensedConsolidatedBalanceSheets" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CondensedConsolidatedStatementsOfCashFlows" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/CondensedConsolidatedStatementsOfOperationsAndComprehensiveIncome" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/Debt" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtCapitalLeasesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtFuturePaymentsUnderTermLoanDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_A1ADC3B941D224051BEB378AA45812A4" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_2E9B8265E7894C3A7733378AA4587773" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A1ADC3B941D224051BEB378AA45812A4" xlink:to="loc_us-gaap_DebtInstrumentTable_2E9B8265E7894C3A7733378AA4587773" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_7612F0A633C99AAB1DFB378AA458762E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_2E9B8265E7894C3A7733378AA4587773" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_7612F0A633C99AAB1DFB378AA458762E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_83204487E1A616BF44AB378AA458ABB0_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_7612F0A633C99AAB1DFB378AA458762E" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_83204487E1A616BF44AB378AA458ABB0_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_83204487E1A616BF44AB378AA458ABB0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_7612F0A633C99AAB1DFB378AA458762E" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_83204487E1A616BF44AB378AA458ABB0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SecuredDebtMember" xlink:label="loc_us-gaap_SecuredDebtMember_6C34932016736247DF93378AA4582E99" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_83204487E1A616BF44AB378AA458ABB0" xlink:to="loc_us-gaap_SecuredDebtMember_6C34932016736247DF93378AA4582E99" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_D0E6C0EEDB575474EDCE378AA458A078" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A1ADC3B941D224051BEB378AA45812A4" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_D0E6C0EEDB575474EDCE378AA458A078" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_C2D1FCAD8D43E6A2109D378AA4582945" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A1ADC3B941D224051BEB378AA45812A4" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_C2D1FCAD8D43E6A2109D378AA4582945" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_4718A62F2FEEE250337E378AA4585077" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A1ADC3B941D224051BEB378AA45812A4" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_4718A62F2FEEE250337E378AA4585077" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour_279F8B490430BE667216378AA4587261" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A1ADC3B941D224051BEB378AA45812A4" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour_279F8B490430BE667216378AA4587261" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive_E386DCFFC475A8A4BF5E378AA45805C9" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A1ADC3B941D224051BEB378AA45812A4" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive_E386DCFFC475A8A4BF5E378AA45805C9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongTermDebt" xlink:label="loc_us-gaap_LongTermDebt_D9138FF80C0CD1CFFCCB378AA458A5C2" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A1ADC3B941D224051BEB378AA45812A4" xlink:to="loc_us-gaap_LongTermDebt_D9138FF80C0CD1CFFCCB378AA458A5C2" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_D1A6A8A85860B686048E378AA461CEC8" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_2599BD3B470C8AE9FE8F378AA46124F6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D1A6A8A85860B686048E378AA461CEC8" xlink:to="loc_us-gaap_DebtInstrumentTable_2599BD3B470C8AE9FE8F378AA46124F6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_3E3B8D88556E29810BAA378AA461D021" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_2599BD3B470C8AE9FE8F378AA46124F6" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_3E3B8D88556E29810BAA378AA461D021" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8256267280037E814285378AA4613780_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_3E3B8D88556E29810BAA378AA461D021" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8256267280037E814285378AA4613780_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8256267280037E814285378AA4613780" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_3E3B8D88556E29810BAA378AA461D021" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8256267280037E814285378AA4613780" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SecuredDebtMember" xlink:label="loc_us-gaap_SecuredDebtMember_433222AB71E5E3D5BBD1378AA46145B0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8256267280037E814285378AA4613780" xlink:to="loc_us-gaap_SecuredDebtMember_433222AB71E5E3D5BBD1378AA46145B0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_1DA9FF42CEFF18C77BB2378AA46184EF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_2599BD3B470C8AE9FE8F378AA46124F6" xlink:to="loc_us-gaap_CreditFacilityAxis_1DA9FF42CEFF18C77BB2378AA46184EF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_0ECABCE657E0A0B0BCB9378AA46179B0_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_1DA9FF42CEFF18C77BB2378AA46184EF" xlink:to="loc_us-gaap_CreditFacilityDomain_0ECABCE657E0A0B0BCB9378AA46179B0_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_0ECABCE657E0A0B0BCB9378AA46179B0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_1DA9FF42CEFF18C77BB2378AA46184EF" xlink:to="loc_us-gaap_CreditFacilityDomain_0ECABCE657E0A0B0BCB9378AA46179B0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_BCCFDBAF58A3A79A3560378AA4610D30" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_0ECABCE657E0A0B0BCB9378AA46179B0" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_BCCFDBAF58A3A79A3560378AA4610D30" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongTermDebt" xlink:label="loc_us-gaap_LongTermDebt_6421202AD20214C6FBD1378AA461DDBC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D1A6A8A85860B686048E378AA461CEC8" xlink:to="loc_us-gaap_LongTermDebt_6421202AD20214C6FBD1378AA461DDBC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LineOfCredit" xlink:label="loc_us-gaap_LineOfCredit_5275B77FEECFDF6D3FF0378AA4619FB2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D1A6A8A85860B686048E378AA461CEC8" xlink:to="loc_us-gaap_LineOfCredit_5275B77FEECFDF6D3FF0378AA4619FB2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_79BC96913045109B908D378AA461BD75" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D1A6A8A85860B686048E378AA461CEC8" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_79BC96913045109B908D378AA461BD75" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet" xlink:label="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_04D629EB00ED18148B9A378AA4611C4E" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D1A6A8A85860B686048E378AA461CEC8" xlink:to="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_04D629EB00ED18148B9A378AA4611C4E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DeferredFinanceCostsNet" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_7BD05ADF95F0BC9541B6378AA462E50B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D1A6A8A85860B686048E378AA461CEC8" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_7BD05ADF95F0BC9541B6378AA462E50B" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtRemainingPaymentsDueOnCapitalLeaseObligationsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DebtTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DiscontinuedOperations" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DiscontinuedOperationsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_4D6A5D00805F6485D91B513E5A1A16DF" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_96C251AE4F0BA8D27544513E5A19FA5D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_4D6A5D00805F6485D91B513E5A1A16DF" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_96C251AE4F0BA8D27544513E5A19FA5D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:label="loc_us-gaap_CounterpartyNameAxis_FDC5F57814140AEA08C3513E5A19591B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_96C251AE4F0BA8D27544513E5A19FA5D" xlink:to="loc_us-gaap_CounterpartyNameAxis_FDC5F57814140AEA08C3513E5A19591B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_6C644D969D63CEAF27A4513E5A1980A0_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CounterpartyNameAxis_FDC5F57814140AEA08C3513E5A19591B" xlink:to="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_6C644D969D63CEAF27A4513E5A1980A0_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_6C644D969D63CEAF27A4513E5A1980A0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CounterpartyNameAxis_FDC5F57814140AEA08C3513E5A19591B" xlink:to="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_6C644D969D63CEAF27A4513E5A1980A0" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_RepublicMember" xlink:label="loc_band_RepublicMember_F80F76C905536F789E2C513E5A1AF69B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_6C644D969D63CEAF27A4513E5A1980A0" xlink:to="loc_band_RepublicMember_F80F76C905536F789E2C513E5A1AF69B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DisposalGroupClassificationAxis" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_D84FC015FB2683BE318D513E5A1AD078" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_96C251AE4F0BA8D27544513E5A19FA5D" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_D84FC015FB2683BE318D513E5A1AD078" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_E223B4560DD04283055B513E5A1A8691_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_D84FC015FB2683BE318D513E5A1AD078" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_E223B4560DD04283055B513E5A1A8691_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_E223B4560DD04283055B513E5A1A8691" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_D84FC015FB2683BE318D513E5A1AD078" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_E223B4560DD04283055B513E5A1A8691" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleSpinoffMember" xlink:label="loc_us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleSpinoffMember_EF3687AF7CE9571FD64D513E5A1A0756" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_E223B4560DD04283055B513E5A1A8691" xlink:to="loc_us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleSpinoffMember_EF3687AF7CE9571FD64D513E5A1A0756" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_CommonStockConversionBasisRatio" xlink:label="loc_band_CommonStockConversionBasisRatio_A1EEAEBBE3EB2F275072513E5A1A7BBA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_4D6A5D00805F6485D91B513E5A1A16DF" xlink:to="loc_band_CommonStockConversionBasisRatio_A1EEAEBBE3EB2F275072513E5A1A7BBA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue_4D369D59C4FC41477A7B513E5A1A9DCF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_4D6A5D00805F6485D91B513E5A1A16DF" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue_4D369D59C4FC41477A7B513E5A1A9DCF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold_4FB0B8C858FAD8D6CB0D513E5A1A054D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_4D6A5D00805F6485D91B513E5A1A16DF" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold_4FB0B8C858FAD8D6CB0D513E5A1A054D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense_B3D65302B30F85CF8BFD513E5A1A80F8" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_4D6A5D00805F6485D91B513E5A1A16DF" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense_B3D65302B30F85CF8BFD513E5A1A80F8" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_DisposalGroupIncludingDiscontinuedOperationDepreciationandInterest" xlink:label="loc_band_DisposalGroupIncludingDiscontinuedOperationDepreciationandInterest_650E92288C22F3C8CF29513E5A1A642B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_4D6A5D00805F6485D91B513E5A1A16DF" xlink:to="loc_band_DisposalGroupIncludingDiscontinuedOperationDepreciationandInterest_650E92288C22F3C8CF29513E5A1A642B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity_8352ED2D60BDD69E930B513E5A1A4B49" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_4D6A5D00805F6485D91B513E5A1A16DF" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity_8352ED2D60BDD69E930B513E5A1A4B49" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DiscontinuedOperationsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/DocumentAndEntityInformationDocumentAndEntityInformation" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_0231F4935D52A61785B72D663168F4A8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_0231F4935D52A61785B72D663168F4A8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_A0273D2B3CDDFDA4FF5E2D6631682245" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_0231F4935D52A61785B72D663168F4A8" xlink:to="loc_us-gaap_StatementClassOfStockAxis_A0273D2B3CDDFDA4FF5E2D6631682245" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_214ECE491015F9BA8CF72D6631688824_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_A0273D2B3CDDFDA4FF5E2D6631682245" xlink:to="loc_us-gaap_ClassOfStockDomain_214ECE491015F9BA8CF72D6631688824_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_214ECE491015F9BA8CF72D6631688824" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_A0273D2B3CDDFDA4FF5E2D6631682245" xlink:to="loc_us-gaap_ClassOfStockDomain_214ECE491015F9BA8CF72D6631688824" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonClassAMember" xlink:label="loc_us-gaap_CommonClassAMember_A6999329D3A3E42AB8712D663168E5C3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_214ECE491015F9BA8CF72D6631688824" xlink:to="loc_us-gaap_CommonClassAMember_A6999329D3A3E42AB8712D663168E5C3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonClassBMember" xlink:label="loc_us-gaap_CommonClassBMember_F456F10CF87D9F3E6AC22D66316898B1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_214ECE491015F9BA8CF72D6631688824" xlink:to="loc_us-gaap_CommonClassBMember_F456F10CF87D9F3E6AC22D66316898B1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityRegistrantName" xlink:label="loc_dei_EntityRegistrantName_FAA8E6D90C24A42876D52D6631698AC6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_EntityRegistrantName_FAA8E6D90C24A42876D52D6631698AC6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCentralIndexKey" xlink:label="loc_dei_EntityCentralIndexKey_FA4C2BB96C594AFC3DDE2D663169B9C7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_EntityCentralIndexKey_FA4C2BB96C594AFC3DDE2D663169B9C7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_CurrentFiscalYearEndDate" xlink:label="loc_dei_CurrentFiscalYearEndDate_8110C417BF959EE608782D6631698C96" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_CurrentFiscalYearEndDate_8110C417BF959EE608782D6631698C96" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityFilerCategory" xlink:label="loc_dei_EntityFilerCategory_ED2719F6002BD188B8972D66316920D1" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_EntityFilerCategory_ED2719F6002BD188B8972D66316920D1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentType" xlink:label="loc_dei_DocumentType_243FC699B028D034E15D2D6631694BC4" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_DocumentType_243FC699B028D034E15D2D6631694BC4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentPeriodEndDate" xlink:label="loc_dei_DocumentPeriodEndDate_5A23A59B646BEBA443CF2D663169ABB3" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_DocumentPeriodEndDate_5A23A59B646BEBA443CF2D663169ABB3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentFiscalYearFocus" xlink:label="loc_dei_DocumentFiscalYearFocus_4AA07EFD55C6E06814732D663169B64A" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_DocumentFiscalYearFocus_4AA07EFD55C6E06814732D663169B64A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="loc_dei_DocumentFiscalPeriodFocus_55726DC477B546C94A952D66316960A2" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_DocumentFiscalPeriodFocus_55726DC477B546C94A952D66316960A2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_AmendmentFlag" xlink:label="loc_dei_AmendmentFlag_86DF5B1B76D016DF95762D6631698A2A" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_AmendmentFlag_86DF5B1B76D016DF95762D6631698A2A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_6A4348054A1C2A6FD7A52D66316AC10D" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E13FCEA8F7BD76AC5742D6631696ACE" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_6A4348054A1C2A6FD7A52D66316AC10D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/EmployeeBenefitPlan" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/EmployeeBenefitPlanDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FairValueOfFinancialInstruments" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FairValueOfFinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_948BE40B2504DD922CB251B5CB2E176B" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_E51489A10801985E5A4A51B5CB2E7808" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_948BE40B2504DD922CB251B5CB2E176B" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_E51489A10801985E5A4A51B5CB2E7808" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_1C0E1BC15B568EBB748E51B5CB2E07AF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_E51489A10801985E5A4A51B5CB2E7808" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_1C0E1BC15B568EBB748E51B5CB2E07AF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_256B025BC8A2044003C151B5CB2E5189_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_1C0E1BC15B568EBB748E51B5CB2E07AF" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_256B025BC8A2044003C151B5CB2E5189_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_256B025BC8A2044003C151B5CB2E5189" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_1C0E1BC15B568EBB748E51B5CB2E07AF" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_256B025BC8A2044003C151B5CB2E5189" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_C4C5D535F663DD64175B51B5CB2EB9E0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_256B025BC8A2044003C151B5CB2E5189" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_C4C5D535F663DD64175B51B5CB2EB9E0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_964168C04DFD6DB79FB951B5CB2E462E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_E51489A10801985E5A4A51B5CB2E7808" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_964168C04DFD6DB79FB951B5CB2E462E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_84D578FBE15147A98BD551B5CB2E5621_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_964168C04DFD6DB79FB951B5CB2E462E" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_84D578FBE15147A98BD551B5CB2E5621_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_84D578FBE15147A98BD551B5CB2E5621" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_964168C04DFD6DB79FB951B5CB2E462E" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_84D578FBE15147A98BD551B5CB2E5621" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherLiabilitiesMember" xlink:label="loc_us-gaap_OtherLiabilitiesMember_681C09F4BD16CC52C02D51B5CB2E521D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_84D578FBE15147A98BD551B5CB2E5621" xlink:to="loc_us-gaap_OtherLiabilitiesMember_681C09F4BD16CC52C02D51B5CB2E521D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_7A3B4309357E3DC4797151B5CB2E69D3" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_E51489A10801985E5A4A51B5CB2E7808" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_7A3B4309357E3DC4797151B5CB2E69D3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_AB327BF5AB8BC883DA6551B5CB2E175A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_7A3B4309357E3DC4797151B5CB2E69D3" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_AB327BF5AB8BC883DA6551B5CB2E175A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_AB327BF5AB8BC883DA6551B5CB2E175A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_7A3B4309357E3DC4797151B5CB2E69D3" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_AB327BF5AB8BC883DA6551B5CB2E175A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_ABBADC41E2AC5477851351B5CB2EBAEA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_AB327BF5AB8BC883DA6551B5CB2E175A" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_ABBADC41E2AC5477851351B5CB2EBAEA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_F34CE7DCFE58C5F94C7C51B5CB2E9A7A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_AB327BF5AB8BC883DA6551B5CB2E175A" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_F34CE7DCFE58C5F94C7C51B5CB2E9A7A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_BC9BE172C74F3FBE378751B5CB2E0BFC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_AB327BF5AB8BC883DA6551B5CB2E175A" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_BC9BE172C74F3FBE378751B5CB2E0BFC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_F981D6E427D2A23595DC51B5CB2F8272" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_948BE40B2504DD922CB251B5CB2E176B" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_F981D6E427D2A23595DC51B5CB2F8272" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherLiabilitiesFairValueDisclosure" xlink:label="loc_us-gaap_OtherLiabilitiesFairValueDisclosure_5B2020F2F877468045DB51B5CB2F91A1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_948BE40B2504DD922CB251B5CB2E176B" xlink:to="loc_us-gaap_OtherLiabilitiesFairValueDisclosure_5B2020F2F877468045DB51B5CB2F91A1" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_ShareholderAgreementTermofNotice" xlink:label="loc_band_ShareholderAgreementTermofNotice_AA82F5DB51E2FCA563F451B5CB2FFF01" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_948BE40B2504DD922CB251B5CB2E176B" xlink:to="loc_band_ShareholderAgreementTermofNotice_AA82F5DB51E2FCA563F451B5CB2FFF01" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_ShareholderAgreementQualifiedPublicOfferingThreshold" xlink:label="loc_band_ShareholderAgreementQualifiedPublicOfferingThreshold_E7017EC85B736A00177651B5CB2FC80A" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_948BE40B2504DD922CB251B5CB2E176B" xlink:to="loc_band_ShareholderAgreementQualifiedPublicOfferingThreshold_E7017EC85B736A00177651B5CB2FC80A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss_2F8BFF022EE00FE14A4451B5CB2F64D0" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_948BE40B2504DD922CB251B5CB2E176B" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss_2F8BFF022EE00FE14A4451B5CB2F64D0" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FairValueOfFinancialInstrumentsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FinancialStatementComponents" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FinancialStatementComponentsAccountsReceivableNetOfAllowanceForDoubtfulAccountsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FinancialStatementComponentsAccruedExpensesAndOtherCurrentLiabilitiesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FinancialStatementComponentsAllowanceForDoubtfulAccountsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccountsNotesAndLoansReceivableLineItems" xlink:label="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_9EDE59BB852E1E8DEB414C9BF80E4290" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable" xlink:label="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_6B081DC6CE66508A78E64C9BF80E71FF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_9EDE59BB852E1E8DEB414C9BF80E4290" xlink:to="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_6B081DC6CE66508A78E64C9BF80E71FF" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_5A7C2F166231D10B4D7E4C9BF80E6D0F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_6B081DC6CE66508A78E64C9BF80E71FF" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_5A7C2F166231D10B4D7E4C9BF80E6D0F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ReceivableTypeDomain" xlink:label="loc_us-gaap_ReceivableTypeDomain_10D628984531E22E513D4C9BF80E0BE2_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_5A7C2F166231D10B4D7E4C9BF80E6D0F" xlink:to="loc_us-gaap_ReceivableTypeDomain_10D628984531E22E513D4C9BF80E0BE2_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ReceivableTypeDomain" xlink:label="loc_us-gaap_ReceivableTypeDomain_10D628984531E22E513D4C9BF80E0BE2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_5A7C2F166231D10B4D7E4C9BF80E6D0F" xlink:to="loc_us-gaap_ReceivableTypeDomain_10D628984531E22E513D4C9BF80E0BE2" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_AccountsReceivableExcludingCarrierAccessBillingCABMember" xlink:label="loc_band_AccountsReceivableExcludingCarrierAccessBillingCABMember_AAE578A10E25141E177B4C9BF80E32E9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_10D628984531E22E513D4C9BF80E0BE2" xlink:to="loc_band_AccountsReceivableExcludingCarrierAccessBillingCABMember_AAE578A10E25141E177B4C9BF80E32E9" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_CarrierAccessBillingMember" xlink:label="loc_band_CarrierAccessBillingMember_40DA76E988D5B16EB9004C9BF80E0D6A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_10D628984531E22E513D4C9BF80E0BE2" xlink:to="loc_band_CarrierAccessBillingMember_40DA76E988D5B16EB9004C9BF80E0D6A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_2DB65FB6EB64A4B1BD3D4C9BF80E80F6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_6B081DC6CE66508A78E64C9BF80E71FF" xlink:to="loc_us-gaap_ProductOrServiceAxis_2DB65FB6EB64A4B1BD3D4C9BF80E80F6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_5B6B363442236563B0244C9BF80EE7B6_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_2DB65FB6EB64A4B1BD3D4C9BF80E80F6" xlink:to="loc_us-gaap_ProductsAndServicesDomain_5B6B363442236563B0244C9BF80EE7B6_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_5B6B363442236563B0244C9BF80EE7B6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_2DB65FB6EB64A4B1BD3D4C9BF80E80F6" xlink:to="loc_us-gaap_ProductsAndServicesDomain_5B6B363442236563B0244C9BF80EE7B6" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_CarrierAccessBillingMember" xlink:label="loc_band_CarrierAccessBillingMember_AA71FAEF4671EBEA83644C9BF80E808D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_5B6B363442236563B0244C9BF80EE7B6" xlink:to="loc_band_CarrierAccessBillingMember_AA71FAEF4671EBEA83644C9BF80E808D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_4AAF7304DF442F184B1C4C9BF80FA8BB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_9EDE59BB852E1E8DEB414C9BF80E4290" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_4AAF7304DF442F184B1C4C9BF80FA8BB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivable" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_5ABDD1FFA4D83F4D9B984C9BF80F7AF9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_4AAF7304DF442F184B1C4C9BF80FA8BB" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_5ABDD1FFA4D83F4D9B984C9BF80F7AF9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProvisionForDoubtfulAccounts" xlink:label="loc_us-gaap_ProvisionForDoubtfulAccounts_877CE69CB6AF23B7C36C4C9BF80FBA5C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_4AAF7304DF442F184B1C4C9BF80FA8BB" xlink:to="loc_us-gaap_ProvisionForDoubtfulAccounts_877CE69CB6AF23B7C36C4C9BF80FBA5C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs_7919AFD5D350A3CCF7DF4C9BF80FBF72" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_4AAF7304DF442F184B1C4C9BF80FA8BB" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs_7919AFD5D350A3CCF7DF4C9BF80FBF72" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GrossTransactionVolume" xlink:label="loc_us-gaap_GrossTransactionVolume_EF779D288EBC2DFC67594C9BF80F10FB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_9EDE59BB852E1E8DEB414C9BF80E4290" xlink:to="loc_us-gaap_GrossTransactionVolume_EF779D288EBC2DFC67594C9BF80F10FB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_60F10B61B115D51613224C9BF80F8A3B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_9EDE59BB852E1E8DEB414C9BF80E4290" xlink:to="loc_us-gaap_Revenues_60F10B61B115D51613224C9BF80F8A3B" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/FinancialStatementComponentsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IncomeTaxes" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IncomeTaxesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IntangibleAssets" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IntangibleAssetsAmortizationExpenseDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IntangibleAssetsFutureEstimatedAmortizationExpenseDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IntangibleAssetsSummaryOfIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_5C9287A018DAF6D4F4C8516F089AEB4A" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_CFD26A1343C5AA8BD866516F089AAC43" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_5C9287A018DAF6D4F4C8516F089AEB4A" xlink:to="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_CFD26A1343C5AA8BD866516F089AAC43" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_E1F0455B6E605B64A259516F089A5EE3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_CFD26A1343C5AA8BD866516F089AAC43" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_E1F0455B6E605B64A259516F089A5EE3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_380B47AF50D8C6553279516F089AB959_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_E1F0455B6E605B64A259516F089A5EE3" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_380B47AF50D8C6553279516F089AB959_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_380B47AF50D8C6553279516F089AB959" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_E1F0455B6E605B64A259516F089A5EE3" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_380B47AF50D8C6553279516F089AB959" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LicensingAgreementsMember" xlink:label="loc_us-gaap_LicensingAgreementsMember_30FF0D6868474BC27B9B516F089ABF26" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_380B47AF50D8C6553279516F089AB959" xlink:to="loc_us-gaap_LicensingAgreementsMember_30FF0D6868474BC27B9B516F089ABF26" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_402A85783D90EB787970516F089A1B23" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_5C9287A018DAF6D4F4C8516F089AEB4A" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_402A85783D90EB787970516F089A1B23" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_30FE790AD375B7B15543516F089BC5DC" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_538D7569888A9F6714A8516F089A8402" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_30FE790AD375B7B15543516F089BC5DC" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_538D7569888A9F6714A8516F089A8402" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_E814DCEBBF3A0E7E70CD516F089A548F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_538D7569888A9F6714A8516F089A8402" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_E814DCEBBF3A0E7E70CD516F089A548F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_E814DCEBBF3A0E7E70CD516F089A548F" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_E814DCEBBF3A0E7E70CD516F089A548F" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_913E6DB73A2C45192B58516F089B1BF6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B" xlink:to="loc_us-gaap_CustomerRelationshipsMember_913E6DB73A2C45192B58516F089B1BF6" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_DomainNameandRelatedTrademarksDomain" xlink:label="loc_band_DomainNameandRelatedTrademarksDomain_41B225AC39A0777A7239516F089BEC62" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B" xlink:to="loc_band_DomainNameandRelatedTrademarksDomain_41B225AC39A0777A7239516F089BEC62" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LicensingAgreementsMember" xlink:label="loc_us-gaap_LicensingAgreementsMember_7C358A5366BA60398C00516F089BDB01" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B" xlink:to="loc_us-gaap_LicensingAgreementsMember_7C358A5366BA60398C00516F089BDB01" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NoncompeteAgreementsMember" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_926657C7959DBC0E5173516F089BC318" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_926657C7959DBC0E5173516F089BC318" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_33CB0FD29FA0AB1363B5516F089B8F83" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4840FC260757278AA659516F089BC67B" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_33CB0FD29FA0AB1363B5516F089B8F83" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_20DA8A0C9697FA695065516F089B2303" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_538D7569888A9F6714A8516F089A8402" xlink:to="loc_us-gaap_RangeAxis_20DA8A0C9697FA695065516F089B2303" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_687099EC12243E6DF49F516F089B6805_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_20DA8A0C9697FA695065516F089B2303" xlink:to="loc_us-gaap_RangeMember_687099EC12243E6DF49F516F089B6805_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_687099EC12243E6DF49F516F089B6805" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_20DA8A0C9697FA695065516F089B2303" xlink:to="loc_us-gaap_RangeMember_687099EC12243E6DF49F516F089B6805" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_DDFB21AB135208054F49516F089B50D7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_687099EC12243E6DF49F516F089B6805" xlink:to="loc_us-gaap_MinimumMember_DDFB21AB135208054F49516F089B50D7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_3AFD07E6D70C17793661516F089B151C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_687099EC12243E6DF49F516F089B6805" xlink:to="loc_us-gaap_MaximumMember_3AFD07E6D70C17793661516F089B151C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_E76DE4014A9B62A651B6516F089BF7E1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_30FE790AD375B7B15543516F089BC5DC" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_E76DE4014A9B62A651B6516F089BF7E1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_D5D0C3092C62167EF02D516F089B84A6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_30FE790AD375B7B15543516F089BC5DC" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_D5D0C3092C62167EF02D516F089B84A6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_623F65F26ADE169BE338516F089B9E95" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_30FE790AD375B7B15543516F089BC5DC" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_623F65F26ADE169BE338516F089B9E95" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_99F3D5CF2D8DF90058A6516F089BE51B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_30FE790AD375B7B15543516F089BC5DC" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_99F3D5CF2D8DF90058A6516F089BE51B" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/IntangibleAssetsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/OrganizationAndDescriptionOfBusiness" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/OrganizationAndDescriptionOfBusinessDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9135A19246894A6ED94B516F08D084C0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9135A19246894A6ED94B516F08D084C0" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_7270826530100C8A097E516F08D05639" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9135A19246894A6ED94B516F08D084C0" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_7270826530100C8A097E516F08D05639" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_C559BE8A274B59ED3F1A516F08D0E264_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_7270826530100C8A097E516F08D05639" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_C559BE8A274B59ED3F1A516F08D0E264_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_C559BE8A274B59ED3F1A516F08D0E264" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_7270826530100C8A097E516F08D05639" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_C559BE8A274B59ED3F1A516F08D0E264" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_C5A616BB141E8BD75515516F08D09E39" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_C559BE8A274B59ED3F1A516F08D0E264" xlink:to="loc_us-gaap_SubsequentEventMember_C5A616BB141E8BD75515516F08D09E39" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_919A77868D2BEF783A38516F08D0B982" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9135A19246894A6ED94B516F08D084C0" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_919A77868D2BEF783A38516F08D0B982" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_3E78B721916983D81E88516F08D015A5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_919A77868D2BEF783A38516F08D0B982" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_3E78B721916983D81E88516F08D015A5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_3E78B721916983D81E88516F08D015A5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_919A77868D2BEF783A38516F08D0B982" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_3E78B721916983D81E88516F08D015A5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IPOMember" xlink:label="loc_us-gaap_IPOMember_D3D41F2D7E301CEBF8EC516F08D0F335" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_3E78B721916983D81E88516F08D015A5" xlink:to="loc_us-gaap_IPOMember_D3D41F2D7E301CEBF8EC516F08D0F335" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_1F84F38F4E69E46F356E516F08D16939" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9135A19246894A6ED94B516F08D084C0" xlink:to="loc_us-gaap_StatementClassOfStockAxis_1F84F38F4E69E46F356E516F08D16939" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_22781F399A290087B66C516F08D15243_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1F84F38F4E69E46F356E516F08D16939" xlink:to="loc_us-gaap_ClassOfStockDomain_22781F399A290087B66C516F08D15243_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_22781F399A290087B66C516F08D15243" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1F84F38F4E69E46F356E516F08D16939" xlink:to="loc_us-gaap_ClassOfStockDomain_22781F399A290087B66C516F08D15243" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonClassBMember" xlink:label="loc_us-gaap_CommonClassBMember_FA26C3AA3A55F9908AAB516F08D1933E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_22781F399A290087B66C516F08D15243" xlink:to="loc_us-gaap_CommonClassBMember_FA26C3AA3A55F9908AAB516F08D1933E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonClassAMember" xlink:label="loc_us-gaap_CommonClassAMember_0629A268FDAE5784382A516F08D1BA89" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_22781F399A290087B66C516F08D15243" xlink:to="loc_us-gaap_CommonClassAMember_0629A268FDAE5784382A516F08D1BA89" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_NewCommonClassADomain" xlink:label="loc_band_NewCommonClassADomain_DE96413B94C015E78008516F08D167BC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_22781F399A290087B66C516F08D15243" xlink:to="loc_band_NewCommonClassADomain_DE96413B94C015E78008516F08D167BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NumberOfOperatingSegments" xlink:label="loc_us-gaap_NumberOfOperatingSegments_9AEEBDC24CFEECEED1A4516F08D11714" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_us-gaap_NumberOfOperatingSegments_9AEEBDC24CFEECEED1A4516F08D11714" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NumberOfReportableSegments" xlink:label="loc_us-gaap_NumberOfReportableSegments_961415B38FFD70CEAB13516F08D167EE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_us-gaap_NumberOfReportableSegments_961415B38FFD70CEAB13516F08D167EE" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_CommonStockConversionBasisRatio" xlink:label="loc_band_CommonStockConversionBasisRatio_497418CE32DA79CB3022516F08D19677" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_band_CommonStockConversionBasisRatio_497418CE32DA79CB3022516F08D19677" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_CommonStockVotingRightsVotesPerShare" xlink:label="loc_band_CommonStockVotingRightsVotesPerShare_56F86C73D61003221DE8516F08D1E7D4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_band_CommonStockVotingRightsVotesPerShare_56F86C73D61003221DE8516F08D1E7D4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_0C40797BE38D8F09D599516F08D147CC" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_0C40797BE38D8F09D599516F08D147CC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_1486B9C94724A2C8A65D516F08D1074C" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_1486B9C94724A2C8A65D516F08D1074C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_103E04FA89CCFDBBFB24516F08D1AD85" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_103E04FA89CCFDBBFB24516F08D1AD85" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_A05376DA802B84DB235B516F08D1BDD4" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6B3D6AA1D544A59B3E09516F08D1D8A0" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_A05376DA802B84DB235B516F08D1BDD4" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/PropertyAndEquipment" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/PropertyAndEquipmentDepreciationExpenseDetails" xlink:type="extended">
    <link:loc xlink:href="band-20170930.xsd#band_DepreciationExpenseLineItems" xlink:label="loc_band_DepreciationExpenseLineItems_73BA0C1B9CECDCC40DE3378AA4A47F0D" xlink:type="locator" />
    <link:loc xlink:href="band-20170930.xsd#band_DepreciationExpenseTable" xlink:label="loc_band_DepreciationExpenseTable_557CE7C0C645ACC2C8A5378AA4A4460F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_band_DepreciationExpenseLineItems_73BA0C1B9CECDCC40DE3378AA4A47F0D" xlink:to="loc_band_DepreciationExpenseTable_557CE7C0C645ACC2C8A5378AA4A4460F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_A1E789E2BBD5D3790AC9378AA4A42132" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_band_DepreciationExpenseTable_557CE7C0C645ACC2C8A5378AA4A4460F" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_A1E789E2BBD5D3790AC9378AA4A42132" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_C9CC26C4A43ACC4C28E6378AA4A41E19_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_A1E789E2BBD5D3790AC9378AA4A42132" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_C9CC26C4A43ACC4C28E6378AA4A41E19_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_C9CC26C4A43ACC4C28E6378AA4A41E19" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_A1E789E2BBD5D3790AC9378AA4A42132" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_C9CC26C4A43ACC4C28E6378AA4A41E19" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CostOfSalesMember" xlink:label="loc_us-gaap_CostOfSalesMember_F9513F9AA8AA18894704378AA4A46CBD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_C9CC26C4A43ACC4C28E6378AA4A41E19" xlink:to="loc_us-gaap_CostOfSalesMember_F9513F9AA8AA18894704378AA4A46CBD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_E10E27CBBD4B041C4120378AA4A41C65" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_C9CC26C4A43ACC4C28E6378AA4A41E19" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_E10E27CBBD4B041C4120378AA4A41C65" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SellingAndMarketingExpenseMember" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_432D0DC232E3B3B3491A378AA4A4085C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_C9CC26C4A43ACC4C28E6378AA4A41E19" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_432D0DC232E3B3B3491A378AA4A4085C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_0AE5C2A512FEDC72742F378AA4A4DF31" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_C9CC26C4A43ACC4C28E6378AA4A41E19" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_0AE5C2A512FEDC72742F378AA4A4DF31" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Depreciation" xlink:label="loc_us-gaap_Depreciation_3824D9493B99085A27C5378AA4A40852" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_band_DepreciationExpenseLineItems_73BA0C1B9CECDCC40DE3378AA4A47F0D" xlink:to="loc_us-gaap_Depreciation_3824D9493B99085A27C5378AA4A40852" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/PropertyAndEquipmentSoftwareDevelopmentCostsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/PropertyAndEquipmentSummaryOfPropertyAndEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_951E5489F249CC6E5F2D378AA4B4CE31" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B1C88F753AC22BC68662378AA4B3EAFB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_951E5489F249CC6E5F2D378AA4B4CE31" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B1C88F753AC22BC68662378AA4B3EAFB" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1DA8BFFAB77F1C66D643378AA4B44540" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B1C88F753AC22BC68662378AA4B3EAFB" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1DA8BFFAB77F1C66D643378AA4B44540" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1DA8BFFAB77F1C66D643378AA4B44540" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1DA8BFFAB77F1C66D643378AA4B44540" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_E1C7F9187BAEC08CE811378AA4B43E60" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_E1C7F9187BAEC08CE811378AA4B43E60" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_ComputerandOfficeEquipmentMember" xlink:label="loc_band_ComputerandOfficeEquipmentMember_3B0A29F2EB7CCA7592E7378AA4B4265D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC" xlink:to="loc_band_ComputerandOfficeEquipmentMember_3B0A29F2EB7CCA7592E7378AA4B4265D" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_TelecommunicationsEquipmentMember" xlink:label="loc_band_TelecommunicationsEquipmentMember_E3825D1610C5AC2C884F378AA4B4AE0C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC" xlink:to="loc_band_TelecommunicationsEquipmentMember_E3825D1610C5AC2C884F378AA4B4AE0C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_6EF8EB6CC297A25E3ABB378AA4B4D56D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_6EF8EB6CC297A25E3ABB378AA4B4D56D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SoftwareDevelopmentMember" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_63F018C35547F29D1645378AA4B4EE17" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_63F018C35547F29D1645378AA4B4EE17" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AutomobilesMember" xlink:label="loc_us-gaap_AutomobilesMember_BF11AF322B3265722A69378AA4B4ABB0" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_A22EEF54EC43D94DD1FD378AA4B4C5EC" xlink:to="loc_us-gaap_AutomobilesMember_BF11AF322B3265722A69378AA4B4ABB0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_CD1DF5E72251A9046E63378AA4B42AC6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_951E5489F249CC6E5F2D378AA4B4CE31" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_CD1DF5E72251A9046E63378AA4B42AC6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_DFB663C8CBCE6559E9BC378AA4B4430F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_951E5489F249CC6E5F2D378AA4B4CE31" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_DFB663C8CBCE6559E9BC378AA4B4430F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_29280586C7D06689F110378AA4B4506E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_951E5489F249CC6E5F2D378AA4B4CE31" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_29280586C7D06689F110378AA4B4506E" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/PropertyAndEquipmentTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/RedeemableConvertiblePreferredStockAndStockholdersDeficitDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_47E3482F048F3D54CA8F516F0874E2F0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_47E3482F048F3D54CA8F516F0874E2F0" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_D76EDC5A52A21366BC33516F0874038C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_47E3482F048F3D54CA8F516F0874E2F0" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_D76EDC5A52A21366BC33516F0874038C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_3294D3D5A3A7484EC8B1516F0874ED0B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_D76EDC5A52A21366BC33516F0874038C" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_3294D3D5A3A7484EC8B1516F0874ED0B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_3294D3D5A3A7484EC8B1516F0874ED0B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_D76EDC5A52A21366BC33516F0874038C" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_3294D3D5A3A7484EC8B1516F0874ED0B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_6DC8131F63DAB815EC82516F087512B5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_3294D3D5A3A7484EC8B1516F0874ED0B" xlink:to="loc_us-gaap_SubsequentEventMember_6DC8131F63DAB815EC82516F087512B5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_078EEF2360BF36DA53C8516F0875B66C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_47E3482F048F3D54CA8F516F0874E2F0" xlink:to="loc_us-gaap_StatementClassOfStockAxis_078EEF2360BF36DA53C8516F0875B66C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_078EEF2360BF36DA53C8516F0875B66C" xlink:to="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_078EEF2360BF36DA53C8516F0875B66C" xlink:to="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_57C199DB6B496AEEE613516F0875DE69" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_57C199DB6B496AEEE613516F0875DE69" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_OldCommonClassAMember" xlink:label="loc_band_OldCommonClassAMember_8D46DDEF3BF3E7909A22516F08754F83" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682" xlink:to="loc_band_OldCommonClassAMember_8D46DDEF3BF3E7909A22516F08754F83" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_OldCommonClassBMember" xlink:label="loc_band_OldCommonClassBMember_E975909A702757228C89516F087536F3" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682" xlink:to="loc_band_OldCommonClassBMember_E975909A702757228C89516F087536F3" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_NewCommonClassBDomain" xlink:label="loc_band_NewCommonClassBDomain_CF709D65E74FC21E2D62516F08753972" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682" xlink:to="loc_band_NewCommonClassBDomain_CF709D65E74FC21E2D62516F08753972" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_NewCommonClassADomain" xlink:label="loc_band_NewCommonClassADomain_83EF8CA059E45EA58C0D516F08754183" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_7AC99A9B220B2A6002A9516F08753682" xlink:to="loc_band_NewCommonClassADomain_83EF8CA059E45EA58C0D516F08754183" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_E63ACFFED7692B92E13E516F08754AE7" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_47E3482F048F3D54CA8F516F0874E2F0" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_E63ACFFED7692B92E13E516F08754AE7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5CA0F051FB7E505D2E66516F0875EB44_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_E63ACFFED7692B92E13E516F08754AE7" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5CA0F051FB7E505D2E66516F0875EB44_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5CA0F051FB7E505D2E66516F0875EB44" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_E63ACFFED7692B92E13E516F08754AE7" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5CA0F051FB7E505D2E66516F0875EB44" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IPOMember" xlink:label="loc_us-gaap_IPOMember_69320F5D4D3D6B9F18CB516F0875857E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5CA0F051FB7E505D2E66516F0875EB44" xlink:to="loc_us-gaap_IPOMember_69320F5D4D3D6B9F18CB516F0875857E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_3B1E413B5D7DB12A4011516F0875ED5E" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_47E3482F048F3D54CA8F516F0874E2F0" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_3B1E413B5D7DB12A4011516F0875ED5E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_53AB9290ACFC930CAB5D516F0875BC06_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_3B1E413B5D7DB12A4011516F0875ED5E" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_53AB9290ACFC930CAB5D516F0875BC06_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_53AB9290ACFC930CAB5D516F0875BC06" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_3B1E413B5D7DB12A4011516F0875ED5E" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_53AB9290ACFC930CAB5D516F0875BC06" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_StockPurchaseWarrantMember" xlink:label="loc_band_StockPurchaseWarrantMember_2F9F28222E14ADA8A2C7516F087544D2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_53AB9290ACFC930CAB5D516F0875BC06" xlink:to="loc_band_StockPurchaseWarrantMember_2F9F28222E14ADA8A2C7516F087544D2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_C9E4CCCFD1C3871E3F4D516F08765B30" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_C9E4CCCFD1C3871E3F4D516F08765B30" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_1D11D48689B76DB0AB04516F0876BCDE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_1D11D48689B76DB0AB04516F0876BCDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="loc_us-gaap_PreferredStockSharesIssued_111B82E996784DC6BE34516F0876FD15" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_PreferredStockSharesIssued_111B82E996784DC6BE34516F0876FD15" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_ConvertiblePreferredStockConversionPrice" xlink:label="loc_band_ConvertiblePreferredStockConversionPrice_25FCD55FD1E5D0810239516F08763321" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_band_ConvertiblePreferredStockConversionPrice_25FCD55FD1E5D0810239516F08763321" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:label="loc_us-gaap_CommonStockSharesIssued_FC298CFF6979873044A1516F08768151" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_CommonStockSharesIssued_FC298CFF6979873044A1516F08768151" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_4827837680EB62E3C952516F0876BBBF" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_4827837680EB62E3C952516F0876BBBF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_BD5BFEB337BD2A1B1BD9516F0876097C" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_BD5BFEB337BD2A1B1BD9516F0876097C" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_CommonStockConversionBasisRatio" xlink:label="loc_band_CommonStockConversionBasisRatio_7FDFDA0C279EA8EB9764516F0876541F" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_band_CommonStockConversionBasisRatio_7FDFDA0C279EA8EB9764516F0876541F" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_CommonStockVotingRightsVotesPerShare" xlink:label="loc_band_CommonStockVotingRightsVotesPerShare_E29523016DA089ED4406516F0876F51D" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_band_CommonStockVotingRightsVotesPerShare_E29523016DA089ED4406516F0876F51D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_A57E562546F1A56CD3FB516F0876387E" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_A57E562546F1A56CD3FB516F0876387E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_CB7A2416D4BF79AEE3AD516F08761494" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_CB7A2416D4BF79AEE3AD516F08761494" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_06F8CFE162E5B2C92AEA516F0876EC39" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_0978B850944FA5EFBC30516F08755FA1" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_06F8CFE162E5B2C92AEA516F0876EC39" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/RedeemableConvertiblePreferredStockAndStockholdersDeficitRedeemableConvertiblePreferredStockAndStockholdersDeficitNotes" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/RelatedParties" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/RelatedPartiesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_CEF3D7A009759F1A02582D9C9F3CF77D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_9C9F947A53D22B2A172A2D9C9F337A98" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_CEF3D7A009759F1A02582D9C9F3CF77D" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_9C9F947A53D22B2A172A2D9C9F337A98" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1BE2ED10836EEB04FB782D9C9F346478" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_9C9F947A53D22B2A172A2D9C9F337A98" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1BE2ED10836EEB04FB782D9C9F346478" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_A02681484A6E36854A0C2D9C9F341D30_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1BE2ED10836EEB04FB782D9C9F346478" xlink:to="loc_us-gaap_RelatedPartyDomain_A02681484A6E36854A0C2D9C9F341D30_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_A02681484A6E36854A0C2D9C9F341D30" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1BE2ED10836EEB04FB782D9C9F346478" xlink:to="loc_us-gaap_RelatedPartyDomain_A02681484A6E36854A0C2D9C9F341D30" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AffiliatedEntityMember" xlink:label="loc_us-gaap_AffiliatedEntityMember_7A92D3EC112568F18AF52D9C9F34C837" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_A02681484A6E36854A0C2D9C9F341D30" xlink:to="loc_us-gaap_AffiliatedEntityMember_7A92D3EC112568F18AF52D9C9F34C837" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:label="loc_us-gaap_CounterpartyNameAxis_BCC66AA31AC6D499A1E12D9C9F341D8D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_9C9F947A53D22B2A172A2D9C9F337A98" xlink:to="loc_us-gaap_CounterpartyNameAxis_BCC66AA31AC6D499A1E12D9C9F341D8D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_F39E8313B64225DA942D2D9C9F348EFA_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CounterpartyNameAxis_BCC66AA31AC6D499A1E12D9C9F341D8D" xlink:to="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_F39E8313B64225DA942D2D9C9F348EFA_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_F39E8313B64225DA942D2D9C9F348EFA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CounterpartyNameAxis_BCC66AA31AC6D499A1E12D9C9F341D8D" xlink:to="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_F39E8313B64225DA942D2D9C9F348EFA" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_RepublicMember" xlink:label="loc_band_RepublicMember_15F50AF9CBFF55278CDB2D9C9F358226" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RepurchaseAgreementCounterpartyNameDomain_F39E8313B64225DA942D2D9C9F348EFA" xlink:to="loc_band_RepublicMember_15F50AF9CBFF55278CDB2D9C9F358226" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_17684644FACC299DA2B82D9C9F3AF7AB" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_9C9F947A53D22B2A172A2D9C9F337A98" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_17684644FACC299DA2B82D9C9F3AF7AB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_473632F1490FD909A4172D9C9F3ADEA6_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_17684644FACC299DA2B82D9C9F3AF7AB" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_473632F1490FD909A4172D9C9F3ADEA6_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_473632F1490FD909A4172D9C9F3ADEA6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_17684644FACC299DA2B82D9C9F3AF7AB" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_473632F1490FD909A4172D9C9F3ADEA6" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_TransitionServicesAgreementMember" xlink:label="loc_band_TransitionServicesAgreementMember_5E8FDC14C20CC03875392D9C9F3A20FF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_473632F1490FD909A4172D9C9F3ADEA6" xlink:to="loc_band_TransitionServicesAgreementMember_5E8FDC14C20CC03875392D9C9F3A20FF" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_FacilitiesSharingAgreementMember" xlink:label="loc_band_FacilitiesSharingAgreementMember_C47A5F08ACDD2368B0F52D9C9F3A4C22" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_473632F1490FD909A4172D9C9F3ADEA6" xlink:to="loc_band_FacilitiesSharingAgreementMember_C47A5F08ACDD2368B0F52D9C9F3A4C22" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_TaxSharingAgreementMember" xlink:label="loc_band_TaxSharingAgreementMember_F9DFD55D6BA6392A278E2D9C9F3B1026" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_473632F1490FD909A4172D9C9F3ADEA6" xlink:to="loc_band_TaxSharingAgreementMember_F9DFD55D6BA6392A278E2D9C9F3B1026" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_MasterServicesAgreementMember" xlink:label="loc_band_MasterServicesAgreementMember_53163E6B383D1C41232D2D9C9F3BE33E" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_473632F1490FD909A4172D9C9F3ADEA6" xlink:to="loc_band_MasterServicesAgreementMember_53163E6B383D1C41232D2D9C9F3BE33E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_3435AB61228D5AA8FB5B2D9C9F3BC855" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_9C9F947A53D22B2A172A2D9C9F337A98" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_3435AB61228D5AA8FB5B2D9C9F3BC855" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_3959B5D0815C7C654EE52D9C9F3B379F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_3435AB61228D5AA8FB5B2D9C9F3BC855" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_3959B5D0815C7C654EE52D9C9F3B379F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_3959B5D0815C7C654EE52D9C9F3B379F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_3435AB61228D5AA8FB5B2D9C9F3BC855" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_3959B5D0815C7C654EE52D9C9F3B379F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_292C4C4C40977B5A63032D9C9F3C8A5A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_3959B5D0815C7C654EE52D9C9F3B379F" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_292C4C4C40977B5A63032D9C9F3C8A5A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RevenueFromRelatedParties" xlink:label="loc_us-gaap_RevenueFromRelatedParties_F4A325ED8246CED761EA2D9C9F3CFE37" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_CEF3D7A009759F1A02582D9C9F3CF77D" xlink:to="loc_us-gaap_RevenueFromRelatedParties_F4A325ED8246CED761EA2D9C9F3CFE37" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccountsReceivableRelatedPartiesCurrent" xlink:label="loc_us-gaap_AccountsReceivableRelatedPartiesCurrent_163CD51D9B3C52BDCCCC2D9C9F3CE564" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_CEF3D7A009759F1A02582D9C9F3CF77D" xlink:to="loc_us-gaap_AccountsReceivableRelatedPartiesCurrent_163CD51D9B3C52BDCCCC2D9C9F3CE564" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue" xlink:label="loc_us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue_71E97B3A75D28E4DEADC2D9C9F3C74D8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_CEF3D7A009759F1A02582D9C9F3CF77D" xlink:to="loc_us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue_71E97B3A75D28E4DEADC2D9C9F3C74D8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DueFromRelatedParties" xlink:label="loc_us-gaap_DueFromRelatedParties_98174D93D1F145C989252D9C9F3CE8C6" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_CEF3D7A009759F1A02582D9C9F3CF77D" xlink:to="loc_us-gaap_DueFromRelatedParties_98174D93D1F145C989252D9C9F3CE8C6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DueToRelatedPartiesCurrentAndNoncurrent" xlink:label="loc_us-gaap_DueToRelatedPartiesCurrentAndNoncurrent_58484209812BCE0DD2CD2D9C9F3C530A" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_CEF3D7A009759F1A02582D9C9F3CF77D" xlink:to="loc_us-gaap_DueToRelatedPartiesCurrentAndNoncurrent_58484209812BCE0DD2CD2D9C9F3C530A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SegmentAndGeographicInformation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SegmentAndGeographicInformationReconciliationOfRevenueByGeographicAreaDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_09FD74E07169F18A51F95068EA9374D8" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_9E413665945A5F4953335068EA92F42C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_09FD74E07169F18A51F95068EA9374D8" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_9E413665945A5F4953335068EA92F42C" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_354ECE2C22E27214B5005068EA92A760" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_9E413665945A5F4953335068EA92F42C" xlink:to="loc_us-gaap_StatementGeographicalAxis_354ECE2C22E27214B5005068EA92A760" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_1A96B603AC70953729B75068EA92EA80_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_354ECE2C22E27214B5005068EA92A760" xlink:to="loc_us-gaap_SegmentGeographicalDomain_1A96B603AC70953729B75068EA92EA80_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_1A96B603AC70953729B75068EA92EA80" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_354ECE2C22E27214B5005068EA92A760" xlink:to="loc_us-gaap_SegmentGeographicalDomain_1A96B603AC70953729B75068EA92EA80" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_US" xlink:label="loc_country_US_7DEEA419C17D8926350E5068EA9357CC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_1A96B603AC70953729B75068EA92EA80" xlink:to="loc_country_US_7DEEA419C17D8926350E5068EA9357CC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NonUsMember" xlink:label="loc_us-gaap_NonUsMember_24621C413C8E003C3FA25068EA932231" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_1A96B603AC70953729B75068EA92EA80" xlink:to="loc_us-gaap_NonUsMember_24621C413C8E003C3FA25068EA932231" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_A6A780E28E1D554BE35E5068EA936E2D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_09FD74E07169F18A51F95068EA9374D8" xlink:to="loc_us-gaap_Revenues_A6A780E28E1D554BE35E5068EA936E2D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SegmentAndGeographicInformationReconciliationOfSegmentProfitLossDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_1DBBDB5D3660123BBA7E507506219B7D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3A0E2DC1294F3E3C751F507506202E6B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_1DBBDB5D3660123BBA7E507506219B7D" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3A0E2DC1294F3E3C751F507506202E6B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_79BA3EE798E5C81D6E57507506203D75" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3A0E2DC1294F3E3C751F507506202E6B" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_79BA3EE798E5C81D6E57507506203D75" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_BD9DE9C29CB0F9B0D341507506201CD5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_79BA3EE798E5C81D6E57507506203D75" xlink:to="loc_us-gaap_SegmentDomain_BD9DE9C29CB0F9B0D341507506201CD5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_BD9DE9C29CB0F9B0D341507506201CD5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_79BA3EE798E5C81D6E57507506203D75" xlink:to="loc_us-gaap_SegmentDomain_BD9DE9C29CB0F9B0D341507506201CD5" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_CPaaSMember" xlink:label="loc_band_CPaaSMember_857CE808245641E9765350750620CE75" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_BD9DE9C29CB0F9B0D341507506201CD5" xlink:to="loc_band_CPaaSMember_857CE808245641E9765350750620CE75" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllOtherSegmentsMember" xlink:label="loc_us-gaap_AllOtherSegmentsMember_491800C4C89E949D6F10507506218427" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_BD9DE9C29CB0F9B0D341507506201CD5" xlink:to="loc_us-gaap_AllOtherSegmentsMember_491800C4C89E949D6F10507506218427" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NumberOfReportableSegments" xlink:label="loc_us-gaap_NumberOfReportableSegments_94F8FACE4FBE9552503A50750621F4D4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_1DBBDB5D3660123BBA7E507506219B7D" xlink:to="loc_us-gaap_NumberOfReportableSegments_94F8FACE4FBE9552503A50750621F4D4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_4CD5DE674680691B054750750621E4CF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_1DBBDB5D3660123BBA7E507506219B7D" xlink:to="loc_us-gaap_Revenues_4CD5DE674680691B054750750621E4CF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CostOfRevenue" xlink:label="loc_us-gaap_CostOfRevenue_F3F089E4699A25A6585550750621E8EC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_1DBBDB5D3660123BBA7E507506219B7D" xlink:to="loc_us-gaap_CostOfRevenue_F3F089E4699A25A6585550750621E8EC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GrossProfit" xlink:label="loc_us-gaap_GrossProfit_FC2FCCDEE9339C00CCBA50750621CCCE" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_1DBBDB5D3660123BBA7E507506219B7D" xlink:to="loc_us-gaap_GrossProfit_FC2FCCDEE9339C00CCBA50750621CCCE" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SegmentAndGeographicInformationTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensationAllocatedStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DB7A8DEC689CBF356CB1513E59A6B5E7" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_02D7F3315936A0494011513E59A54E1A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DB7A8DEC689CBF356CB1513E59A6B5E7" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_02D7F3315936A0494011513E59A54E1A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_D54BB1DB17D20FB8863E513E59A5425D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_02D7F3315936A0494011513E59A54E1A" xlink:to="loc_us-gaap_AwardTypeAxis_D54BB1DB17D20FB8863E513E59A5425D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7ED9D610FC60B6323CCB513E59A506B2_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_D54BB1DB17D20FB8863E513E59A5425D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7ED9D610FC60B6323CCB513E59A506B2_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7ED9D610FC60B6323CCB513E59A506B2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_D54BB1DB17D20FB8863E513E59A5425D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7ED9D610FC60B6323CCB513E59A506B2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_4E6EC6D7059FE961ACBB513E59A55AE3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7ED9D610FC60B6323CCB513E59A506B2" xlink:to="loc_us-gaap_EmployeeStockOptionMember_4E6EC6D7059FE961ACBB513E59A55AE3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_634C71C842D316A2E245513E59A53623" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_02D7F3315936A0494011513E59A54E1A" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_634C71C842D316A2E245513E59A53623" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_68116A04EA649CD3BCBD513E59A58091_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_634C71C842D316A2E245513E59A53623" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_68116A04EA649CD3BCBD513E59A58091_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_68116A04EA649CD3BCBD513E59A58091" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_634C71C842D316A2E245513E59A53623" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_68116A04EA649CD3BCBD513E59A58091" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CostOfSalesMember" xlink:label="loc_us-gaap_CostOfSalesMember_CE321383B853A486C48E513E59A5EFD6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_68116A04EA649CD3BCBD513E59A58091" xlink:to="loc_us-gaap_CostOfSalesMember_CE321383B853A486C48E513E59A5EFD6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_2E95A7CADF078459A8AC513E59A51D19" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_68116A04EA649CD3BCBD513E59A58091" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_2E95A7CADF078459A8AC513E59A51D19" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SellingAndMarketingExpenseMember" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_12789A6240E9A721E7C5513E59A643AE" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_68116A04EA649CD3BCBD513E59A58091" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_12789A6240E9A721E7C5513E59A643AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_7668B8B1A173289B9213513E59A640BB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_68116A04EA649CD3BCBD513E59A58091" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_7668B8B1A173289B9213513E59A640BB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_DB364A26A02432984043513E59A60878" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DB7A8DEC689CBF356CB1513E59A6B5E7" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_DB364A26A02432984043513E59A60878" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_1507BA85461F99111C96513E59A65310" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DB7A8DEC689CBF356CB1513E59A6B5E7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_1507BA85461F99111C96513E59A65310" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_4C3D6FD8CA5F3E663E40513E59A6260B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DB7A8DEC689CBF356CB1513E59A6B5E7" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_4C3D6FD8CA5F3E663E40513E59A6260B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_6E38095F26857C6FF71A513E59A60386" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DB7A8DEC689CBF356CB1513E59A6B5E7" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_6E38095F26857C6FF71A513E59A60386" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensationDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2720BA50210444581B88513E59B6DDBE" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8B7AD63AF917380AACCC513E59B6CCC6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2720BA50210444581B88513E59B6DDBE" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8B7AD63AF917380AACCC513E59B6CCC6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_24CCC65FBDF255986A6D513E59B66830" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8B7AD63AF917380AACCC513E59B6CCC6" xlink:to="loc_us-gaap_PlanNameAxis_24CCC65FBDF255986A6D513E59B66830" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_75D52CC35259980D3E71513E59B6CDCC_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_24CCC65FBDF255986A6D513E59B66830" xlink:to="loc_us-gaap_PlanNameDomain_75D52CC35259980D3E71513E59B6CDCC_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_75D52CC35259980D3E71513E59B6CDCC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_24CCC65FBDF255986A6D513E59B66830" xlink:to="loc_us-gaap_PlanNameDomain_75D52CC35259980D3E71513E59B6CDCC" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_A2010EquityCompensationPlanMember" xlink:label="loc_band_A2010EquityCompensationPlanMember_46514102D91AA1C57695513E59B66AD0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_75D52CC35259980D3E71513E59B6CDCC" xlink:to="loc_band_A2010EquityCompensationPlanMember_46514102D91AA1C57695513E59B66AD0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_E3178C8117258CAF7F66513E59B6EE17" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8B7AD63AF917380AACCC513E59B6CCC6" xlink:to="loc_us-gaap_AwardTypeAxis_E3178C8117258CAF7F66513E59B6EE17" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B791AA49B50A9161723F513E59B63436_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_E3178C8117258CAF7F66513E59B6EE17" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B791AA49B50A9161723F513E59B63436_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B791AA49B50A9161723F513E59B63436" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_E3178C8117258CAF7F66513E59B6EE17" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B791AA49B50A9161723F513E59B63436" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_42777444BFD61C934EA1513E59B6388E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B791AA49B50A9161723F513E59B63436" xlink:to="loc_us-gaap_EmployeeStockOptionMember_42777444BFD61C934EA1513E59B6388E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_B8DDB88A02B450F69A82513E59B6C62F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2720BA50210444581B88513E59B6DDBE" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_B8DDB88A02B450F69A82513E59B6C62F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_FFF1FE79BD6D5A7D88EF513E59B76121" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2720BA50210444581B88513E59B6DDBE" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_FFF1FE79BD6D5A7D88EF513E59B76121" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_8154B9C010837CA66029513E59B7E55F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2720BA50210444581B88513E59B6DDBE" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_8154B9C010837CA66029513E59B7E55F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_5E651FC583028F5B2B33513E59B7100E" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2720BA50210444581B88513E59B6DDBE" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_5E651FC583028F5B2B33513E59B7100E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_2AE76C3ACAC9BEE68EAA513E59B732DA" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2720BA50210444581B88513E59B6DDBE" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_2AE76C3ACAC9BEE68EAA513E59B732DA" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensationFairValueOfOptionsGrantedUsingBlackScholesPricingModelDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_DC464259BB562737A077513E59B11609" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_DC464259BB562737A077513E59B11609" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_A8A4979BF8D140A6BE64513E59B1A91E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_DC464259BB562737A077513E59B11609" xlink:to="loc_us-gaap_AwardTypeAxis_A8A4979BF8D140A6BE64513E59B1A91E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BD698CD798D181DED1B513E59B11F77_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_A8A4979BF8D140A6BE64513E59B1A91E" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BD698CD798D181DED1B513E59B11F77_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BD698CD798D181DED1B513E59B11F77" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_A8A4979BF8D140A6BE64513E59B1A91E" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BD698CD798D181DED1B513E59B11F77" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_4551D11B0E56B20BFA94513E59B1C15B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BD698CD798D181DED1B513E59B11F77" xlink:to="loc_us-gaap_EmployeeStockOptionMember_4551D11B0E56B20BFA94513E59B1C15B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_F467F3DA9659A6331285513E59B14887" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_DC464259BB562737A077513E59B11609" xlink:to="loc_us-gaap_RangeAxis_F467F3DA9659A6331285513E59B14887" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_81B93B63F1898FF79BDD513E59B1D1FB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_F467F3DA9659A6331285513E59B14887" xlink:to="loc_us-gaap_RangeMember_81B93B63F1898FF79BDD513E59B1D1FB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_81B93B63F1898FF79BDD513E59B1D1FB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_F467F3DA9659A6331285513E59B14887" xlink:to="loc_us-gaap_RangeMember_81B93B63F1898FF79BDD513E59B1D1FB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_C8CE882C0FCC53E3CE0C513E59B16566" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_81B93B63F1898FF79BDD513E59B1D1FB" xlink:to="loc_us-gaap_MinimumMember_C8CE882C0FCC53E3CE0C513E59B16566" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_EB958ACFB05AFBCBDE6C513E59B25598" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_81B93B63F1898FF79BDD513E59B1D1FB" xlink:to="loc_us-gaap_MaximumMember_EB958ACFB05AFBCBDE6C513E59B25598" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_DCCB09C014F532EDB319513E59B2C7EA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_DCCB09C014F532EDB319513E59B2C7EA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_985125AEF0113D1D0DC1513E59B2F74F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_985125AEF0113D1D0DC1513E59B2F74F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_2869F4FE49C437978CDB513E59B2B90E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_2869F4FE49C437978CDB513E59B2B90E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_31484AE72582A816CBC6513E59B2DD7E" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_31484AE72582A816CBC6513E59B2DD7E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_B256768AFA23437C37F6513E59B2D26E" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_B256768AFA23437C37F6513E59B2D26E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_1EAE0DCF32DFE5ACEB1D513E59B2C4C0" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_1EAE0DCF32DFE5ACEB1D513E59B2C4C0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_0CED7BE823BF1295C87F513E59B24F54" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_0CED7BE823BF1295C87F513E59B24F54" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_SharebasedCompensationArrangementbySharebasedPaymentAwardFairValueAssumptionsFairValueRange" xlink:label="loc_band_SharebasedCompensationArrangementbySharebasedPaymentAwardFairValueAssumptionsFairValueRange_771A8ECB96D7D0D49382513E59B2BA80" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_39B9E1BBDCECB610F81B513E59B24E0D" xlink:to="loc_band_SharebasedCompensationArrangementbySharebasedPaymentAwardFairValueAssumptionsFairValueRange_771A8ECB96D7D0D49382513E59B2BA80" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensationStockOptionActivityDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/StockBasedCompensationTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SubsequentEvents" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:label="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTable" xlink:label="loc_us-gaap_SubsequentEventTable_94A3D131F42B38552383513E596BF8DC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_SubsequentEventTable_94A3D131F42B38552383513E596BF8DC" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_EBDAE0B3E366EF8BFDF9513E596B809A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_94A3D131F42B38552383513E596BF8DC" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_EBDAE0B3E366EF8BFDF9513E596B809A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_7828FFCD72BAE363DDE1513E596BE675_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_EBDAE0B3E366EF8BFDF9513E596B809A" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_7828FFCD72BAE363DDE1513E596BE675_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_7828FFCD72BAE363DDE1513E596BE675" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_EBDAE0B3E366EF8BFDF9513E596B809A" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_7828FFCD72BAE363DDE1513E596BE675" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_4D145E76B1E0ABC7AC29513E596BA190" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_7828FFCD72BAE363DDE1513E596BE675" xlink:to="loc_us-gaap_SubsequentEventMember_4D145E76B1E0ABC7AC29513E596BA190" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_2B1125F885D4B71EC1BF513E596B7593" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_94A3D131F42B38552383513E596BF8DC" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_2B1125F885D4B71EC1BF513E596B7593" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_7E621AE98CC2663A85D2513E596B25C0_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_2B1125F885D4B71EC1BF513E596B7593" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_7E621AE98CC2663A85D2513E596B25C0_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_7E621AE98CC2663A85D2513E596B25C0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_2B1125F885D4B71EC1BF513E596B7593" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_7E621AE98CC2663A85D2513E596B25C0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IPOMember" xlink:label="loc_us-gaap_IPOMember_B9B6E6F071D70AEB738C513E596B8E49" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_7E621AE98CC2663A85D2513E596B25C0" xlink:to="loc_us-gaap_IPOMember_B9B6E6F071D70AEB738C513E596B8E49" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OverAllotmentOptionMember" xlink:label="loc_us-gaap_OverAllotmentOptionMember_053E23A1C5893105DA9E513E596BA92B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_7E621AE98CC2663A85D2513E596B25C0" xlink:to="loc_us-gaap_OverAllotmentOptionMember_053E23A1C5893105DA9E513E596BA92B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_973DC8868B082F2A0E77513E596BE65D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_94A3D131F42B38552383513E596BF8DC" xlink:to="loc_us-gaap_StatementClassOfStockAxis_973DC8868B082F2A0E77513E596BE65D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_973DC8868B082F2A0E77513E596BE65D" xlink:to="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_973DC8868B082F2A0E77513E596BE65D" xlink:to="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_OldCommonClassAMember" xlink:label="loc_band_OldCommonClassAMember_F24ED662016B70768AE8513E596B0893" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0" xlink:to="loc_band_OldCommonClassAMember_F24ED662016B70768AE8513E596B0893" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_OldCommonClassBMember" xlink:label="loc_band_OldCommonClassBMember_CB00E417D61D3BAA82FB513E596B40AB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0" xlink:to="loc_band_OldCommonClassBMember_CB00E417D61D3BAA82FB513E596B40AB" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_NewCommonClassADomain" xlink:label="loc_band_NewCommonClassADomain_72DDB2D37A50B0663AB1513E596CFB8D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0" xlink:to="loc_band_NewCommonClassADomain_72DDB2D37A50B0663AB1513E596CFB8D" xlink:type="arc" />
    <link:loc xlink:href="band-20170930.xsd#band_NewCommonClassBDomain" xlink:label="loc_band_NewCommonClassBDomain_A15AC01646C0D56C7427513E596CF45D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0" xlink:to="loc_band_NewCommonClassBDomain_A15AC01646C0D56C7427513E596CF45D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_97C859922F00737912D6513E596C0ACC" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_97C859922F00737912D6513E596C0ACC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonClassAMember" xlink:label="loc_us-gaap_CommonClassAMember_033022ACC494BEA5DC74513E596C2318" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0BA83981D2E43BEA23B9513E596BDBE0" xlink:to="loc_us-gaap_CommonClassAMember_033022ACC494BEA5DC74513E596C2318" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_6282B4E1AC1E0FB5577B513E596C7177" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_94A3D131F42B38552383513E596BF8DC" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_6282B4E1AC1E0FB5577B513E596C7177" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_55E1408C60F3945FA54C513E596C5EE1_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_6282B4E1AC1E0FB5577B513E596C7177" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_55E1408C60F3945FA54C513E596C5EE1_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_55E1408C60F3945FA54C513E596C5EE1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_6282B4E1AC1E0FB5577B513E596C7177" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_55E1408C60F3945FA54C513E596C5EE1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SecuredDebtMember" xlink:label="loc_us-gaap_SecuredDebtMember_1AA4218E29458E72D111513E596CDE26" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_55E1408C60F3945FA54C513E596C5EE1" xlink:to="loc_us-gaap_SecuredDebtMember_1AA4218E29458E72D111513E596CDE26" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_E07E98846EB3766CCBB8513E596CC5D0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_E07E98846EB3766CCBB8513E596CC5D0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_85AF32ECD88341672A42513E596C38ED" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_85AF32ECD88341672A42513E596C38ED" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_41B56A6840C540B01C40513E596CB882" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_41B56A6840C540B01C40513E596CB882" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_F0B6B34EB567B42BD361513E596C7952" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_F0B6B34EB567B42BD361513E596C7952" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_02DC47EFBF0230ADEF6F513E596C69C7" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_02DC47EFBF0230ADEF6F513E596C69C7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_4F1AE560BD8D0F496980513E596CDFBE" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_4F1AE560BD8D0F496980513E596CDFBE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_DB9F5F8E35A4D96676AF513E596C0596" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_DB9F5F8E35A4D96676AF513E596C0596" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PaymentsOfStockIssuanceCosts" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_17DF9B0A9519D43146F0513E596C1C8F" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_17DF9B0A9519D43146F0513E596C1C8F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_9B1CB1AFDDC05EB06841513E596CC9F0" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_9B1CB1AFDDC05EB06841513E596CC9F0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepaymentsOfLongTermDebt" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_7AAE9104688FE3E65C4B513E596C458F" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_7AAE9104688FE3E65C4B513E596C458F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_8B3EA7CBB6D404295F57513E596C9533" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084AB37DCD867B0F9ACE513E596CC34A" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_8B3EA7CBB6D404295F57513E596C9533" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SummaryOfSignificantAccountingPolicies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.bandwidth.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="extended" />
</link:linkbase>
