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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e40b9-d018-7c8c-a1d2-6ae3b9329688,g:902ea157-75c0-479c-a47f-6fd21578be37-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.protolabs.com/role/ConsolidatedStatementsofShareholdersEquity" xlink:type="simple" xlink:href="prlb-20260630.xsd#ConsolidatedStatementsofShareholdersEquity"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/ConsolidatedStatementsofShareholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e40b9-d102-72b0-a0d8-525ab9fd8c04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e40b9-d102-7743-b58c-c819891e5b9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e40b9-d102-72b0-a0d8-525ab9fd8c04" xlink:to="loc_us-gaap_StatementTable_019e40b9-d102-7743-b58c-c819891e5b9c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e40b9-d102-7b03-80fc-6b46155de131" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e40b9-d102-7743-b58c-c819891e5b9c" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e40b9-d102-7b03-80fc-6b46155de131" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e40b9-d102-7b03-80fc-6b46155de131_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e40b9-d102-7b03-80fc-6b46155de131" xlink:to="loc_us-gaap_EquityComponentDomain_019e40b9-d102-7b03-80fc-6b46155de131_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e40b9-d102-73a7-93ab-44e906904244" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e40b9-d102-7b03-80fc-6b46155de131" xlink:to="loc_us-gaap_EquityComponentDomain_019e40b9-d102-73a7-93ab-44e906904244" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e40b9-d102-7be9-876d-52e80682332a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e40b9-d102-73a7-93ab-44e906904244" xlink:to="loc_us-gaap_CommonStockMember_019e40b9-d102-7be9-876d-52e80682332a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e40b9-d102-7204-8700-83af937a99ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e40b9-d102-73a7-93ab-44e906904244" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e40b9-d102-7204-8700-83af937a99ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e40b9-d102-710c-8a4c-fcdef11088a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e40b9-d102-73a7-93ab-44e906904244" xlink:to="loc_us-gaap_RetainedEarningsMember_019e40b9-d102-710c-8a4c-fcdef11088a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e40b9-d102-7706-9ad7-975970db3b6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e40b9-d102-73a7-93ab-44e906904244" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e40b9-d102-7706-9ad7-975970db3b6d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e40b9-d102-72b0-a0d8-525ab9fd8c04" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e40b9-d102-7d76-ae01-6980146b0978" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e40b9-d102-7d76-ae01-6980146b0978" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e40b9-d102-78d1-9bfd-9e8ac4899221" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_StockholdersEquity_019e40b9-d102-78d1-9bfd-9e8ac4899221" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesOther_019e40b9-d102-704b-b87c-356eddccf0dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesOther_019e40b9-d102-704b-b87c-356eddccf0dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueOther_019e40b9-d102-7820-8441-0d56e748a233" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueOther_019e40b9-d102-7820-8441-0d56e748a233" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e40b9-d102-799f-ba58-52fa5bbc162e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e40b9-d102-799f-ba58-52fa5bbc162e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e40b9-d102-77ba-adb2-ceadb265f4d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e40b9-d102-77ba-adb2-ceadb265f4d3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019e40b9-d102-7f32-b2ea-0de92525e668" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019e40b9-d102-7f32-b2ea-0de92525e668" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e40b9-d102-71aa-b7cc-62b1df1b8f3f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_NetIncomeLoss_019e40b9-d102-71aa-b7cc-62b1df1b8f3f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019e40b9-d102-707e-a73b-833840456fec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019e40b9-d102-707e-a73b-833840456fec" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_019e40b9-d102-7edf-8c23-ed39904a242b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019e40b9-d102-707e-a73b-833840456fec" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_019e40b9-d102-7edf-8c23-ed39904a242b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_019e40b9-d102-712a-8f33-adb35058c2db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_019e40b9-d102-707e-a73b-833840456fec" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_019e40b9-d102-712a-8f33-adb35058c2db" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_019e40b9-d102-751d-b58f-da612747cfc1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e40b9-d102-7063-b377-8f4efd7cec08" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_019e40b9-d102-751d-b58f-da612747cfc1" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e40b9-d102-7891-924c-655492bfe1db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e40b9-d102-7a10-bf3b-1744d348e607" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/GoodwillandOtherIntangibleAssetsNarrativeDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#GoodwillandOtherIntangibleAssetsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/GoodwillandOtherIntangibleAssetsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-782a-ba56-0fd402aa95a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e40b9-d102-7b97-b750-745466c4755b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-782a-ba56-0fd402aa95a8" xlink:to="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e40b9-d102-7b97-b750-745466c4755b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019e40b9-d102-73b6-a83d-d672d28ccd83" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e40b9-d102-7b97-b750-745466c4755b" xlink:to="loc_srt_StatementGeographicalAxis_019e40b9-d102-73b6-a83d-d672d28ccd83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e40b9-d102-73b6-a83d-d672d28ccd83_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019e40b9-d102-73b6-a83d-d672d28ccd83" xlink:to="loc_srt_SegmentGeographicalDomain_019e40b9-d102-73b6-a83d-d672d28ccd83_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e40b9-d102-7997-a7cf-effc0cd1f57f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019e40b9-d102-73b6-a83d-d672d28ccd83" xlink:to="loc_srt_SegmentGeographicalDomain_019e40b9-d102-7997-a7cf-effc0cd1f57f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EuropeMember_019e40b9-d102-7497-a022-f2799fd5537b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e40b9-d102-7997-a7cf-effc0cd1f57f" xlink:to="loc_srt_EuropeMember_019e40b9-d102-7497-a022-f2799fd5537b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019e40b9-d102-71e7-adf5-1b3ed0836334" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e40b9-d102-7997-a7cf-effc0cd1f57f" xlink:to="loc_country_US_019e40b9-d102-71e7-adf5-1b3ed0836334" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019e40b9-d102-74d7-88f6-f95d3908083d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e40b9-d102-7b97-b750-745466c4755b" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019e40b9-d102-74d7-88f6-f95d3908083d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e40b9-d102-74d7-88f6-f95d3908083d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019e40b9-d102-74d7-88f6-f95d3908083d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e40b9-d102-74d7-88f6-f95d3908083d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e40b9-d102-7472-9e0e-e114ebe34850" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019e40b9-d102-74d7-88f6-f95d3908083d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e40b9-d102-7472-9e0e-e114ebe34850" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_A3DHubsInc.HubsMember_019e40b9-d102-7be2-9d8a-affae4159961" xlink:href="prlb-20260630.xsd#prlb_A3DHubsInc.HubsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e40b9-d102-7472-9e0e-e114ebe34850" xlink:to="loc_prlb_A3DHubsInc.HubsMember_019e40b9-d102-7be2-9d8a-affae4159961" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPeriodIncreaseDecrease_019e40b9-d102-7c44-8533-130655403832" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillPeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-782a-ba56-0fd402aa95a8" xlink:to="loc_us-gaap_GoodwillPeriodIncreaseDecrease_019e40b9-d102-7c44-8533-130655403832" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019e40b9-d102-73c2-92c4-b146b195b875" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-782a-ba56-0fd402aa95a8" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019e40b9-d102-73c2-92c4-b146b195b875" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentBeforeTax_019e40b9-d102-7ac3-a476-c88c041b6d05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-782a-ba56-0fd402aa95a8" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentBeforeTax_019e40b9-d102-7ac3-a476-c88c041b6d05" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_019e40b9-d102-774c-99ed-bb5a0b0afb90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-782a-ba56-0fd402aa95a8" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_019e40b9-d102-774c-99ed-bb5a0b0afb90" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsOtherThanGoodwillDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsOtherThanGoodwillDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsOtherThanGoodwillDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-746b-9ba0-57a65b55bd1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e40b9-d102-76a5-a9b1-89c8f34e6e5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-746b-9ba0-57a65b55bd1b" xlink:to="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e40b9-d102-76a5-a9b1-89c8f34e6e5f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e40b9-d102-7ec1-a30c-f962dd0c6968" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e40b9-d102-76a5-a9b1-89c8f34e6e5f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e40b9-d102-7ec1-a30c-f962dd0c6968" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e40b9-d102-7ec1-a30c-f962dd0c6968_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e40b9-d102-7ec1-a30c-f962dd0c6968" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e40b9-d102-7ec1-a30c-f962dd0c6968_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e40b9-d102-7226-bbf1-bdac2cf806aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e40b9-d102-7ec1-a30c-f962dd0c6968" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e40b9-d102-7226-bbf1-bdac2cf806aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019e40b9-d102-7e29-9282-54f1100c0ba4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e40b9-d102-7226-bbf1-bdac2cf806aa" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019e40b9-d102-7e29-9282-54f1100c0ba4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareInternallyDevelopedAndPurchasedMember_019efa4a-0a94-742f-a19b-897275363395" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareInternallyDevelopedAndPurchasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e40b9-d102-7226-bbf1-bdac2cf806aa" xlink:to="loc_us-gaap_SoftwareInternallyDevelopedAndPurchasedMember_019efa4a-0a94-742f-a19b-897275363395" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_SoftwarePlatformMember_019e40b9-d102-76c4-850f-b169b8cf767b" xlink:href="prlb-20260630.xsd#prlb_SoftwarePlatformMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e40b9-d102-7226-bbf1-bdac2cf806aa" xlink:to="loc_prlb_SoftwarePlatformMember_019e40b9-d102-76c4-850f-b169b8cf767b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e40b9-d102-7d8b-a757-62aad1255016" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e40b9-d102-76a5-a9b1-89c8f34e6e5f" xlink:to="loc_srt_RangeAxis_019e40b9-d102-7d8b-a757-62aad1255016" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e40b9-d102-7d8b-a757-62aad1255016_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e40b9-d102-7d8b-a757-62aad1255016" xlink:to="loc_srt_RangeMember_019e40b9-d102-7d8b-a757-62aad1255016_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e40b9-d102-7814-bde7-6fe4c6d8a86f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e40b9-d102-7d8b-a757-62aad1255016" xlink:to="loc_srt_RangeMember_019e40b9-d102-7814-bde7-6fe4c6d8a86f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e40b9-d102-7181-9fc5-3e28491ec414" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e40b9-d102-7814-bde7-6fe4c6d8a86f" xlink:to="loc_srt_MinimumMember_019e40b9-d102-7181-9fc5-3e28491ec414" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e40b9-d102-7917-9091-1b249ffe854b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e40b9-d102-7814-bde7-6fe4c6d8a86f" xlink:to="loc_srt_MaximumMember_019e40b9-d102-7917-9091-1b249ffe854b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e40b9-d102-78d0-8d68-55aa9758b950" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-746b-9ba0-57a65b55bd1b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e40b9-d102-78d0-8d68-55aa9758b950" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e40b9-d102-7d5f-90c3-8692872d6170" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-746b-9ba0-57a65b55bd1b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e40b9-d102-7d5f-90c3-8692872d6170" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e40b9-d102-7ac2-8ea8-0f57eb0a4245" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-746b-9ba0-57a65b55bd1b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e40b9-d102-7ac2-8ea8-0f57eb0a4245" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019e40b9-d102-74b8-abad-2c356c81a244" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-746b-9ba0-57a65b55bd1b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019e40b9-d102-74b8-abad-2c356c81a244" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_019e40b9-d102-7da6-99e7-850b5aec6134" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e40b9-d102-746b-9ba0-57a65b55bd1b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_019e40b9-d102-7da6-99e7-850b5aec6134" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/FairValueMeasurementsDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#FairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/FairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e40b9-d102-7f44-8a44-2dd00cfd93de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e40b9-d102-7eb7-bc7f-c19d2f6798ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e40b9-d102-7f44-8a44-2dd00cfd93de" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e40b9-d102-7eb7-bc7f-c19d2f6798ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019e40b9-d102-7b68-9faa-d0cec674d856" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e40b9-d102-7eb7-bc7f-c19d2f6798ac" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019e40b9-d102-7b68-9faa-d0cec674d856" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e40b9-d102-7b68-9faa-d0cec674d856_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e40b9-d102-7b68-9faa-d0cec674d856" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e40b9-d102-7b68-9faa-d0cec674d856_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e40b9-d102-7432-b1de-2019bae9e5c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e40b9-d102-7b68-9faa-d0cec674d856" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e40b9-d102-7432-b1de-2019bae9e5c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashMember_019e40b9-d102-70e7-8aa8-6fe1df5441aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e40b9-d102-7432-b1de-2019bae9e5c7" xlink:to="loc_us-gaap_CashMember_019e40b9-d102-70e7-8aa8-6fe1df5441aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019e40b9-d102-7a53-8d5a-fe7958413d6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e40b9-d102-7432-b1de-2019bae9e5c7" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019e40b9-d102-7a53-8d5a-fe7958413d6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e40b9-d102-7731-a6b4-5b196d92b6ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e40b9-d102-7eb7-bc7f-c19d2f6798ac" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e40b9-d102-7731-a6b4-5b196d92b6ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e40b9-d102-7731-a6b4-5b196d92b6ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e40b9-d102-7731-a6b4-5b196d92b6ba" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e40b9-d102-7731-a6b4-5b196d92b6ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e40b9-d102-7daf-ac11-586205397751" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e40b9-d102-7731-a6b4-5b196d92b6ba" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e40b9-d102-7daf-ac11-586205397751" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019e40b9-d102-7676-a95e-1c1b3cbd6c01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e40b9-d102-7daf-ac11-586205397751" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019e40b9-d102-7676-a95e-1c1b3cbd6c01" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e40b9-d102-7514-a2ac-a7d7b272b810" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e40b9-d102-7daf-ac11-586205397751" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e40b9-d102-7514-a2ac-a7d7b272b810" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e40b9-d102-7a62-8500-1ba721fbdc04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e40b9-d102-7daf-ac11-586205397751" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e40b9-d102-7a62-8500-1ba721fbdc04" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e40b9-d102-79da-9b23-64f37ed012d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e40b9-d102-7f44-8a44-2dd00cfd93de" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e40b9-d102-79da-9b23-64f37ed012d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e40b9-d102-70bd-9629-95d1c13ab49c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e40b9-d102-79da-9b23-64f37ed012d8" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e40b9-d102-70bd-9629-95d1c13ab49c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e40b9-d102-7777-b762-66b9a03586c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e40b9-d102-79da-9b23-64f37ed012d8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e40b9-d102-7777-b762-66b9a03586c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019e40b9-d102-79d1-bad3-3e2284a33301" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e40b9-d102-79da-9b23-64f37ed012d8" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019e40b9-d102-79d1-bad3-3e2284a33301" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/MarketableSecuritiesScheduleofShortTermandLongTermMarketableSecuritiesDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#MarketableSecuritiesScheduleofShortTermandLongTermMarketableSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/MarketableSecuritiesScheduleofShortTermandLongTermMarketableSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e40b9-d102-7fa6-88b9-c242322d50ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e40b9-d102-79c9-b210-69d3efd02a13" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e40b9-d102-7fa6-88b9-c242322d50ce" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e40b9-d102-79c9-b210-69d3efd02a13" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e40b9-d102-7502-a642-9ca7b2876361" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e40b9-d102-79c9-b210-69d3efd02a13" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e40b9-d102-7502-a642-9ca7b2876361" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e40b9-d102-7502-a642-9ca7b2876361_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e40b9-d102-7502-a642-9ca7b2876361" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e40b9-d102-7502-a642-9ca7b2876361_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e40b9-d102-7e11-9999-1da093ade0b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e40b9-d102-7502-a642-9ca7b2876361" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e40b9-d102-7e11-9999-1da093ade0b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e40b9-d102-7702-952f-cb836d3c6255" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e40b9-d102-7e11-9999-1da093ade0b6" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e40b9-d102-7702-952f-cb836d3c6255" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e40b9-d102-75ab-9f4f-1138af491ff1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e40b9-d102-7e11-9999-1da093ade0b6" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e40b9-d102-75ab-9f4f-1138af491ff1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019e40b9-d102-7387-99de-f0d2fff34397" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e40b9-d102-7e11-9999-1da093ade0b6" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019e40b9-d102-7387-99de-f0d2fff34397" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryBondSecuritiesMember_019e40b9-d102-78b5-99a2-ac1180494adc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasuryBondSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e40b9-d102-7e11-9999-1da093ade0b6" xlink:to="loc_us-gaap_USTreasuryBondSecuritiesMember_019e40b9-d102-78b5-99a2-ac1180494adc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019e40b9-d102-7ce7-aeee-d75b3743cf21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e40b9-d102-7fa6-88b9-c242322d50ce" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019e40b9-d102-7ce7-aeee-d75b3743cf21" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e40b9-d102-77fd-8f54-35c2a191e0d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e40b9-d102-7fa6-88b9-c242322d50ce" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e40b9-d102-77fd-8f54-35c2a191e0d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e40b9-d102-746c-8eb3-0a7a66618a07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e40b9-d102-7fa6-88b9-c242322d50ce" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e40b9-d102-746c-8eb3-0a7a66618a07" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e40b9-d102-7e53-824f-d4c8e2aab2a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e40b9-d102-7fa6-88b9-c242322d50ce" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e40b9-d102-7e53-824f-d4c8e2aab2a5" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-7636-bfb3-68ff2c94e571" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-7636-bfb3-68ff2c94e571" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019e40b9-d102-7699-945e-efe3fce9b3be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-7636-bfb3-68ff2c94e571" xlink:to="loc_us-gaap_PlanNameAxis_019e40b9-d102-7699-945e-efe3fce9b3be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e40b9-d102-7699-945e-efe3fce9b3be_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019e40b9-d102-7699-945e-efe3fce9b3be" xlink:to="loc_us-gaap_PlanNameDomain_019e40b9-d102-7699-945e-efe3fce9b3be_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e40b9-d102-7182-9efb-0384eba738f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019e40b9-d102-7699-945e-efe3fce9b3be" xlink:to="loc_us-gaap_PlanNameDomain_019e40b9-d102-7182-9efb-0384eba738f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_LongTermIncentivePlan2022Member_019e40b9-d102-7b45-8448-dacd4e648f0d" xlink:href="prlb-20260630.xsd#prlb_LongTermIncentivePlan2022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e40b9-d102-7182-9efb-0384eba738f9" xlink:to="loc_prlb_LongTermIncentivePlan2022Member_019e40b9-d102-7b45-8448-dacd4e648f0d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7229-9eb9-3c3ed6606369" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-7636-bfb3-68ff2c94e571" xlink:to="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7229-9eb9-3c3ed6606369" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7229-9eb9-3c3ed6606369_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7229-9eb9-3c3ed6606369" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7229-9eb9-3c3ed6606369_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7ef4-9fb7-fb3323ea3354" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7229-9eb9-3c3ed6606369" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7ef4-9fb7-fb3323ea3354" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_ESPPMember_019e40b9-d102-7e89-8a9e-f6f2190ae408" xlink:href="prlb-20260630.xsd#prlb_ESPPMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7ef4-9fb7-fb3323ea3354" xlink:to="loc_prlb_ESPPMember_019e40b9-d102-7e89-8a9e-f6f2190ae408" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e40b9-d102-75da-8ba8-e2960b3b51b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7ef4-9fb7-fb3323ea3354" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e40b9-d102-75da-8ba8-e2960b3b51b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e40b9-d102-7579-a09d-3294d7f4ac0e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7ef4-9fb7-fb3323ea3354" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e40b9-d102-7579-a09d-3294d7f4ac0e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e40b9-d102-7612-8d52-b9f0d59293ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7ef4-9fb7-fb3323ea3354" xlink:to="loc_us-gaap_PerformanceSharesMember_019e40b9-d102-7612-8d52-b9f0d59293ac" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e40b9-d102-729b-bbeb-2841270fbb77" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-7636-bfb3-68ff2c94e571" xlink:to="loc_srt_RangeAxis_019e40b9-d102-729b-bbeb-2841270fbb77" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e40b9-d102-729b-bbeb-2841270fbb77_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e40b9-d102-729b-bbeb-2841270fbb77" xlink:to="loc_srt_RangeMember_019e40b9-d102-729b-bbeb-2841270fbb77_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e40b9-d102-7f46-965e-88f48235ffb3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e40b9-d102-729b-bbeb-2841270fbb77" xlink:to="loc_srt_RangeMember_019e40b9-d102-7f46-965e-88f48235ffb3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e40b9-d102-7c53-bcbc-56215541ebdd" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e40b9-d102-7f46-965e-88f48235ffb3" xlink:to="loc_srt_MinimumMember_019e40b9-d102-7c53-bcbc-56215541ebdd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e40b9-d102-703b-a40a-be7aac0b63d3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e40b9-d102-7f46-965e-88f48235ffb3" xlink:to="loc_srt_MaximumMember_019e40b9-d102-703b-a40a-be7aac0b63d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019e40b9-d102-73fe-a2da-fe5c499bdead" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019e40b9-d102-73fe-a2da-fe5c499bdead" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019e40b9-d102-74d4-9286-4500f307522b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019e40b9-d102-74d4-9286-4500f307522b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate_019e40b9-d102-7841-8c37-ef3bae772e69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate_019e40b9-d102-7841-8c37-ef3bae772e69" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_EmployeeStockPurchasePlanOfferingPeriod_019e40b9-d102-7826-b02e-99a923bac9f6" xlink:href="prlb-20260630.xsd#prlb_EmployeeStockPurchasePlanOfferingPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_prlb_EmployeeStockPurchasePlanOfferingPeriod_019e40b9-d102-7826-b02e-99a923bac9f6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_019e40b9-d102-7302-9cac-3207d9086cc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_019e40b9-d102-7302-9cac-3207d9086cc9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e40b9-d102-7962-97c7-52e57c551c4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e40b9-d102-7962-97c7-52e57c551c4c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e40b9-d102-79be-a044-34ac8a22117f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e40b9-d102-79be-a044-34ac8a22117f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e40b9-d102-73be-bead-6541f28cf74b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e40b9-d102-73be-bead-6541f28cf74b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e40b9-d102-7482-88f1-abc1bc38343c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e40b9-d102-7482-88f1-abc1bc38343c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e40b9-d102-7597-9a5d-11be8756f819" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e40b9-d102-7597-9a5d-11be8756f819" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_PercentageOfTargetNumberCapableOfBeingEarnedAndVestingLowerLimit_019e40b9-d102-7f0b-af2d-001e691a9364" xlink:href="prlb-20260630.xsd#prlb_PercentageOfTargetNumberCapableOfBeingEarnedAndVestingLowerLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_prlb_PercentageOfTargetNumberCapableOfBeingEarnedAndVestingLowerLimit_019e40b9-d102-7f0b-af2d-001e691a9364" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_PercentageOfTargetNumberCapableOfBeingEarnedAndVestingUpperLimit_019e40b9-d102-70a9-9307-1d09bd9f5d2f" xlink:href="prlb-20260630.xsd#prlb_PercentageOfTargetNumberCapableOfBeingEarnedAndVestingUpperLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-79e3-a8e2-ae841142aece" xlink:to="loc_prlb_PercentageOfTargetNumberCapableOfBeingEarnedAndVestingUpperLimit_019e40b9-d102-70a9-9307-1d09bd9f5d2f" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/StockBasedCompensationScheduleofAssumptionsUsedinBlackScholesAndMonteCarloPricingModelDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#StockBasedCompensationScheduleofAssumptionsUsedinBlackScholesAndMonteCarloPricingModelDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/StockBasedCompensationScheduleofAssumptionsUsedinBlackScholesAndMonteCarloPricingModelDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-79f3-b64f-1789fcc47af5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-79f3-b64f-1789fcc47af5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7b8b-83cd-4ba8dd71a0d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-79f3-b64f-1789fcc47af5" xlink:to="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7b8b-83cd-4ba8dd71a0d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7b8b-83cd-4ba8dd71a0d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7b8b-83cd-4ba8dd71a0d4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7b8b-83cd-4ba8dd71a0d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7470-9fb4-5bcec8685188" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7b8b-83cd-4ba8dd71a0d4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7470-9fb4-5bcec8685188" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e40b9-d102-75be-a707-9210d4d0513e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7470-9fb4-5bcec8685188" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e40b9-d102-75be-a707-9210d4d0513e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e40b9-d102-7157-9a37-8198a472e2d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7470-9fb4-5bcec8685188" xlink:to="loc_us-gaap_PerformanceSharesMember_019e40b9-d102-7157-9a37-8198a472e2d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019e40b9-d102-7dd5-a50b-93510056d33b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-79f3-b64f-1789fcc47af5" xlink:to="loc_us-gaap_PlanNameAxis_019e40b9-d102-7dd5-a50b-93510056d33b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e40b9-d102-7dd5-a50b-93510056d33b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019e40b9-d102-7dd5-a50b-93510056d33b" xlink:to="loc_us-gaap_PlanNameDomain_019e40b9-d102-7dd5-a50b-93510056d33b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e40b9-d102-78b7-aee3-06e78cb27ba5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019e40b9-d102-7dd5-a50b-93510056d33b" xlink:to="loc_us-gaap_PlanNameDomain_019e40b9-d102-78b7-aee3-06e78cb27ba5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_EmployeeStockPurchasePlanMember_019e40b9-d102-70d4-98c5-cdc7e4b01b06" xlink:href="prlb-20260630.xsd#prlb_EmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e40b9-d102-78b7-aee3-06e78cb27ba5" xlink:to="loc_prlb_EmployeeStockPurchasePlanMember_019e40b9-d102-70d4-98c5-cdc7e4b01b06" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e6c38-6949-7765-adf9-6ac8efd66ff3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-79f3-b64f-1789fcc47af5" xlink:to="loc_srt_RangeAxis_019e6c38-6949-7765-adf9-6ac8efd66ff3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6c38-6949-7765-adf9-6ac8efd66ff3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e6c38-6949-7765-adf9-6ac8efd66ff3" xlink:to="loc_srt_RangeMember_019e6c38-6949-7765-adf9-6ac8efd66ff3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6c38-6949-7b04-835e-2c0cbc292e72" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e6c38-6949-7765-adf9-6ac8efd66ff3" xlink:to="loc_srt_RangeMember_019e6c38-6949-7b04-835e-2c0cbc292e72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e6c38-a0b3-7b9a-812a-f43898776a9d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6c38-6949-7b04-835e-2c0cbc292e72" xlink:to="loc_srt_MinimumMember_019e6c38-a0b3-7b9a-812a-f43898776a9d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e6c38-ff0b-7991-82ca-0f125f543a28" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6c38-6949-7b04-835e-2c0cbc292e72" xlink:to="loc_srt_MaximumMember_019e6c38-ff0b-7991-82ca-0f125f543a28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_019e6c3d-5458-75ef-9981-7cdfa3870428" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_019e6c3d-5458-75ef-9981-7cdfa3870428" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_019e6c3d-ab94-7877-85f7-afa8156eba12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_019e6c3d-ab94-7877-85f7-afa8156eba12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e40b9-d102-7345-8333-e171c56b059c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e40b9-d102-7345-8333-e171c56b059c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e40b9-d102-7918-8ab8-a06dac78a478" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e40b9-d102-7918-8ab8-a06dac78a478" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_019e6c41-6b4c-7dce-84bf-c41f29a6dcfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_019e6c41-6b4c-7dce-84bf-c41f29a6dcfd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_019e6c41-f5dc-7055-b938-95540c2a79be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_019e6c41-f5dc-7055-b938-95540c2a79be" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e40b9-d102-74c0-85ab-1d2d23fdc7ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e40b9-d102-74c0-85ab-1d2d23fdc7ef" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e40b9-d102-7eba-8c13-6bc74ec9bbce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-712d-ba22-d06269599ef4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e40b9-d102-7eba-8c13-6bc74ec9bbce" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/StockBasedCompensationScheduleofRestrictedAndPerformanceStockUnitActivityDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#StockBasedCompensationScheduleofRestrictedAndPerformanceStockUnitActivityDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/StockBasedCompensationScheduleofRestrictedAndPerformanceStockUnitActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-7e39-8a3b-800444074529" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-7273-b305-a449ec684797" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-7e39-8a3b-800444074529" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-7273-b305-a449ec684797" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7fbc-a8f6-e2d892c90be8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e40b9-d102-7273-b305-a449ec684797" xlink:to="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7fbc-a8f6-e2d892c90be8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7fbc-a8f6-e2d892c90be8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7fbc-a8f6-e2d892c90be8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7fbc-a8f6-e2d892c90be8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7f2a-8e28-329a73f54b05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e40b9-d102-7fbc-a8f6-e2d892c90be8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7f2a-8e28-329a73f54b05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e40b9-d102-7abd-9124-a6e89794c1ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7f2a-8e28-329a73f54b05" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e40b9-d102-7abd-9124-a6e89794c1ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e40b9-d102-7f22-b25c-57fb7544b11f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e40b9-d102-7f2a-8e28-329a73f54b05" xlink:to="loc_us-gaap_PerformanceSharesMember_019e40b9-d102-7f22-b25c-57fb7544b11f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e40b9-d102-762e-ba46-754a7f332c54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-7e39-8a3b-800444074529" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e40b9-d102-762e-ba46-754a7f332c54" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e40b9-d102-709c-98fa-68c0ce17ea3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e40b9-d102-762e-ba46-754a7f332c54" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e40b9-d102-709c-98fa-68c0ce17ea3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e40b9-d102-7d81-bbaa-54334a8b363d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e40b9-d102-762e-ba46-754a7f332c54" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e40b9-d102-7d81-bbaa-54334a8b363d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e40b9-d102-7802-b982-3615780eaea1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e40b9-d102-762e-ba46-754a7f332c54" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e40b9-d102-7802-b982-3615780eaea1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceChange_019e40b9-d102-7910-88ae-cea36a2bad91" xlink:href="prlb-20260630.xsd#prlb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceChange"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e40b9-d102-762e-ba46-754a7f332c54" xlink:to="loc_prlb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceChange_019e40b9-d102-7910-88ae-cea36a2bad91" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e40b9-d102-75aa-b4ee-d882c07f7c3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e40b9-d102-762e-ba46-754a7f332c54" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e40b9-d102-75aa-b4ee-d882c07f7c3e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e40b9-d102-778c-a1a5-d0cafb862b1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e40b9-d102-7f78-af1a-6537cefdd4a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e40b9-d102-7e39-8a3b-800444074529" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e40b9-d102-7f78-af1a-6537cefdd4a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e40b9-d102-7881-82c0-a2fefb05e978" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e40b9-d102-7f78-af1a-6537cefdd4a0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e40b9-d102-7881-82c0-a2fefb05e978" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e40b9-d102-747e-9489-b139b5853113" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e40b9-d102-7f78-af1a-6537cefdd4a0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e40b9-d102-747e-9489-b139b5853113" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e40b9-d102-74d3-97a6-e241a7e62057" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e40b9-d102-7f78-af1a-6537cefdd4a0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e40b9-d102-74d3-97a6-e241a7e62057" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceChangeWeightedAverageGrantDateFairValue_019e40b9-d102-789a-a400-e77bfdaba61b" xlink:href="prlb-20260630.xsd#prlb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceChangeWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e40b9-d102-7f78-af1a-6537cefdd4a0" xlink:to="loc_prlb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceChangeWeightedAverageGrantDateFairValue_019e40b9-d102-789a-a400-e77bfdaba61b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e40b9-d102-7046-9f99-e2d68552258e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e40b9-d102-7f78-af1a-6537cefdd4a0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e40b9-d102-7046-9f99-e2d68552258e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e40b9-d102-7583-8abb-30f1d13040db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/AccumulatedOtherComprehensiveLossIncomeDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#AccumulatedOtherComprehensiveLossIncomeDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/AccumulatedOtherComprehensiveLossIncomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e40b9-d102-70f8-8cad-b3b15d2cf197" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e40b9-d102-772d-b613-f304d50576d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e40b9-d102-70f8-8cad-b3b15d2cf197" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e40b9-d102-772d-b613-f304d50576d6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e40b9-d102-744c-b747-5d23117f875c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e40b9-d102-772d-b613-f304d50576d6" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e40b9-d102-744c-b747-5d23117f875c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e40b9-d102-744c-b747-5d23117f875c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e40b9-d102-744c-b747-5d23117f875c" xlink:to="loc_us-gaap_EquityComponentDomain_019e40b9-d102-744c-b747-5d23117f875c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e40b9-d102-7f69-81ca-415b8d5764d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e40b9-d102-744c-b747-5d23117f875c" xlink:to="loc_us-gaap_EquityComponentDomain_019e40b9-d102-7f69-81ca-415b8d5764d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e40b9-d102-7c69-ab3d-671639497682" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e40b9-d102-7f69-81ca-415b8d5764d5" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e40b9-d102-7c69-ab3d-671639497682" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_019e40b9-d102-7185-86da-fb069d43038a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e40b9-d102-7c69-ab3d-671639497682" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_019e40b9-d102-7185-86da-fb069d43038a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019e40b9-d102-70eb-ad3b-b0613f30b196" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e40b9-d102-7c69-ab3d-671639497682" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019e40b9-d102-70eb-ad3b-b0613f30b196" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e40b9-d102-704f-b0af-f87101788fc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AOCIAttributableToParentNetOfTaxRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e40b9-d102-70f8-8cad-b3b15d2cf197" xlink:to="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e40b9-d102-704f-b0af-f87101788fc9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e40b9-d102-7a9a-a01b-6ccce3f0ea34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e40b9-d102-704f-b0af-f87101788fc9" xlink:to="loc_us-gaap_StockholdersEquity_019e40b9-d102-7a9a-a01b-6ccce3f0ea34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParentAbstract_019e40b9-d102-7470-b2c6-fb9a82820e6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e40b9-d102-704f-b0af-f87101788fc9" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParentAbstract_019e40b9-d102-7470-b2c6-fb9a82820e6e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent_019e40b9-d102-724e-8bc5-46be43973895" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParentAbstract_019e40b9-d102-7470-b2c6-fb9a82820e6e" xlink:to="loc_us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent_019e40b9-d102-724e-8bc5-46be43973895" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent_019e40b9-d102-7175-a8ed-303eadaf744f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParentAbstract_019e40b9-d102-7470-b2c6-fb9a82820e6e" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent_019e40b9-d102-7175-a8ed-303eadaf744f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract_019e40b9-d102-7668-993e-8be0688f1398" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e40b9-d102-704f-b0af-f87101788fc9" xlink:to="loc_us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract_019e40b9-d102-7668-993e-8be0688f1398" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent_019e40b9-d102-7bfe-bbb0-ca986f3eaa74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent_019e40b9-d102-7f8b-afc1-20c56f8a8ec6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e40b9-d102-7389-809b-cd6e561e3573" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e40b9-d102-704f-b0af-f87101788fc9" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e40b9-d102-7389-809b-cd6e561e3573" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e40b9-d102-7d97-b99d-697110c4fd37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/SegmentReportingScheduleofReportableSegmentsDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#SegmentReportingScheduleofReportableSegmentsDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/SegmentReportingScheduleofReportableSegmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7055-a1f6-19e4afcf2d99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-73ac-824c-5daa402610e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7055-a1f6-19e4afcf2d99" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-73ac-824c-5daa402610e4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7bc2-be14-c62412df807f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-73ac-824c-5daa402610e4" xlink:to="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7bc2-be14-c62412df807f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7bc2-be14-c62412df807f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7bc2-be14-c62412df807f" xlink:to="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7bc2-be14-c62412df807f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7b44-8163-ca8b26af0bfb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7bc2-be14-c62412df807f" xlink:to="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7b44-8163-ca8b26af0bfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e40b9-d102-70a6-95bf-848c19a365a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7b44-8163-ca8b26af0bfb" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e40b9-d102-70a6-95bf-848c19a365a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_019e40b9-d102-79aa-af5e-4880b926e578" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7b44-8163-ca8b26af0bfb" xlink:to="loc_us-gaap_CorporateNonSegmentMember_019e40b9-d102-79aa-af5e-4880b926e578" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-7787-9694-64b84022cffb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-73ac-824c-5daa402610e4" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-7787-9694-64b84022cffb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e40b9-d102-7787-9694-64b84022cffb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-7787-9694-64b84022cffb" xlink:to="loc_us-gaap_SegmentDomain_019e40b9-d102-7787-9694-64b84022cffb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e40b9-d102-70a9-8447-d6ce1c0ec680" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-7787-9694-64b84022cffb" xlink:to="loc_us-gaap_SegmentDomain_019e40b9-d102-70a9-8447-d6ce1c0ec680" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_UnitedStatesSegmentMember_019e40b9-d102-7c7f-adb8-1f12b13889b3" xlink:href="prlb-20260630.xsd#prlb_UnitedStatesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e40b9-d102-70a9-8447-d6ce1c0ec680" xlink:to="loc_prlb_UnitedStatesSegmentMember_019e40b9-d102-7c7f-adb8-1f12b13889b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_EuropeSegmentMember_019e40b9-d102-7b16-a216-594f887a07e7" xlink:href="prlb-20260630.xsd#prlb_EuropeSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e40b9-d102-70a9-8447-d6ce1c0ec680" xlink:to="loc_prlb_EuropeSegmentMember_019e40b9-d102-7b16-a216-594f887a07e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019e40b9-d102-7383-a91a-a1fd66114e10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7055-a1f6-19e4afcf2d99" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019e40b9-d102-7383-a91a-a1fd66114e10" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingOtherItemAmount_019e40b9-d102-7b21-9a85-8198345bff30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingOtherItemAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7055-a1f6-19e4afcf2d99" xlink:to="loc_us-gaap_SegmentReportingOtherItemAmount_019e40b9-d102-7b21-9a85-8198345bff30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCosts_019e40b9-d102-7a22-8edd-d1a4bb46cb7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7055-a1f6-19e4afcf2d99" xlink:to="loc_us-gaap_RestructuringCosts_019e40b9-d102-7a22-8edd-d1a4bb46cb7d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_019e6c14-e7ab-7094-ae9d-156df0d84b33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossOnSaleOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7055-a1f6-19e4afcf2d99" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_019e6c14-e7ab-7094-ae9d-156df0d84b33" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019e40b9-d102-723c-8cd0-ded31511bed0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7055-a1f6-19e4afcf2d99" xlink:to="loc_us-gaap_OperatingIncomeLoss_019e40b9-d102-723c-8cd0-ded31511bed0" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/SegmentReportingScheduleofLongLivedAssetsbyGeographicAreasDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#SegmentReportingScheduleofLongLivedAssetsbyGeographicAreasDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/SegmentReportingScheduleofLongLivedAssetsbyGeographicAreasDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-79c4-8aa6-d2df045ec833" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7729-847d-4ac35e6bd48c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-79c4-8aa6-d2df045ec833" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7729-847d-4ac35e6bd48c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e40b9-d102-763e-93ad-b4d790ad71a0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7729-847d-4ac35e6bd48c" xlink:to="loc_srt_ConsolidationItemsAxis_019e40b9-d102-763e-93ad-b4d790ad71a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e40b9-d102-763e-93ad-b4d790ad71a0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e40b9-d102-763e-93ad-b4d790ad71a0" xlink:to="loc_srt_ConsolidationItemsDomain_019e40b9-d102-763e-93ad-b4d790ad71a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7c73-9ac5-a398272ff1eb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e40b9-d102-763e-93ad-b4d790ad71a0" xlink:to="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7c73-9ac5-a398272ff1eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e40b9-d102-7335-8278-ca79f969ecc2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7c73-9ac5-a398272ff1eb" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e40b9-d102-7335-8278-ca79f969ecc2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-77ff-8129-f4cab340bf42" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7729-847d-4ac35e6bd48c" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-77ff-8129-f4cab340bf42" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e40b9-d102-77ff-8129-f4cab340bf42_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-77ff-8129-f4cab340bf42" xlink:to="loc_us-gaap_SegmentDomain_019e40b9-d102-77ff-8129-f4cab340bf42_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e40b9-d102-7a85-a93c-19ded0b5b932" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-77ff-8129-f4cab340bf42" xlink:to="loc_us-gaap_SegmentDomain_019e40b9-d102-7a85-a93c-19ded0b5b932" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_UnitedStatesSegmentMember_019e40b9-d102-74ce-99d2-637349cdb346" xlink:href="prlb-20260630.xsd#prlb_UnitedStatesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e40b9-d102-7a85-a93c-19ded0b5b932" xlink:to="loc_prlb_UnitedStatesSegmentMember_019e40b9-d102-74ce-99d2-637349cdb346" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_EuropeSegmentMember_019e40b9-d102-75be-ab30-a0df597f8145" xlink:href="prlb-20260630.xsd#prlb_EuropeSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e40b9-d102-7a85-a93c-19ded0b5b932" xlink:to="loc_prlb_EuropeSegmentMember_019e40b9-d102-75be-ab30-a0df597f8145" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019e40b9-d102-763f-a833-79d340de2024" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-79c4-8aa6-d2df045ec833" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019e40b9-d102-763f-a833-79d340de2024" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/SegmentReportingScheduleofExpendituresforAdditionstoLonglivedAssetsbyGeographicRegionDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#SegmentReportingScheduleofExpendituresforAdditionstoLonglivedAssetsbyGeographicRegionDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/SegmentReportingScheduleofExpendituresforAdditionstoLonglivedAssetsbyGeographicRegionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-76f8-bbe3-91fba38f45fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7895-b4bb-c0993ec174b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-76f8-bbe3-91fba38f45fb" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7895-b4bb-c0993ec174b3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7067-b97a-a85c01c05b9b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7895-b4bb-c0993ec174b3" xlink:to="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7067-b97a-a85c01c05b9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7067-b97a-a85c01c05b9b_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7067-b97a-a85c01c05b9b" xlink:to="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7067-b97a-a85c01c05b9b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7bc0-b90d-7eaa11345dee" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7067-b97a-a85c01c05b9b" xlink:to="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7bc0-b90d-7eaa11345dee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e40b9-d102-74b7-bb81-984360b59dec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7bc0-b90d-7eaa11345dee" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e40b9-d102-74b7-bb81-984360b59dec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-70e7-9609-6b7cdbadafed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7895-b4bb-c0993ec174b3" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-70e7-9609-6b7cdbadafed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e40b9-d102-70e7-9609-6b7cdbadafed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-70e7-9609-6b7cdbadafed" xlink:to="loc_us-gaap_SegmentDomain_019e40b9-d102-70e7-9609-6b7cdbadafed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e40b9-d102-764e-bc1a-c409a277a20b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-70e7-9609-6b7cdbadafed" xlink:to="loc_us-gaap_SegmentDomain_019e40b9-d102-764e-bc1a-c409a277a20b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_UnitedStatesSegmentMember_019e40b9-d102-7367-a787-de298870fe48" xlink:href="prlb-20260630.xsd#prlb_UnitedStatesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e40b9-d102-764e-bc1a-c409a277a20b" xlink:to="loc_prlb_UnitedStatesSegmentMember_019e40b9-d102-7367-a787-de298870fe48" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_EuropeSegmentMember_019e40b9-d102-7022-9cec-893177934fbf" xlink:href="prlb-20260630.xsd#prlb_EuropeSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e40b9-d102-764e-bc1a-c409a277a20b" xlink:to="loc_prlb_EuropeSegmentMember_019e40b9-d102-7022-9cec-893177934fbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019e40b9-d102-7aac-abbc-67f3ed8df32e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-76f8-bbe3-91fba38f45fb" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019e40b9-d102-7aac-abbc-67f3ed8df32e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/SegmentReportingScheduleofDepreciationandAmortizationbyGeographicRegionDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#SegmentReportingScheduleofDepreciationandAmortizationbyGeographicRegionDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/SegmentReportingScheduleofDepreciationandAmortizationbyGeographicRegionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7d2c-82ca-054168ff7b8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-746d-916c-17c8426644d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7d2c-82ca-054168ff7b8e" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-746d-916c-17c8426644d8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7f4c-86f0-b0ae0c099f3a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-746d-916c-17c8426644d8" xlink:to="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7f4c-86f0-b0ae0c099f3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7f4c-86f0-b0ae0c099f3a_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7f4c-86f0-b0ae0c099f3a" xlink:to="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7f4c-86f0-b0ae0c099f3a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7a61-90dd-ac8eaeef35c4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e40b9-d102-7f4c-86f0-b0ae0c099f3a" xlink:to="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7a61-90dd-ac8eaeef35c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e40b9-d102-79be-ba82-1a44f97146ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7a61-90dd-ac8eaeef35c4" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e40b9-d102-79be-ba82-1a44f97146ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_019e40b9-d102-7a0a-a3d6-900fe77909bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e40b9-d102-7a61-90dd-ac8eaeef35c4" xlink:to="loc_us-gaap_CorporateNonSegmentMember_019e40b9-d102-7a0a-a3d6-900fe77909bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-7e52-8477-6c574ced2e38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-746d-916c-17c8426644d8" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-7e52-8477-6c574ced2e38" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e40b9-d102-7e52-8477-6c574ced2e38_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-7e52-8477-6c574ced2e38" xlink:to="loc_us-gaap_SegmentDomain_019e40b9-d102-7e52-8477-6c574ced2e38_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e40b9-d102-7565-8361-1bbc3c19656e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e40b9-d102-7e52-8477-6c574ced2e38" xlink:to="loc_us-gaap_SegmentDomain_019e40b9-d102-7565-8361-1bbc3c19656e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_UnitedStatesSegmentMember_019e40b9-d102-7bed-9754-0a867d8ac2f0" xlink:href="prlb-20260630.xsd#prlb_UnitedStatesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e40b9-d102-7565-8361-1bbc3c19656e" xlink:to="loc_prlb_UnitedStatesSegmentMember_019e40b9-d102-7bed-9754-0a867d8ac2f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_EuropeSegmentMember_019e40b9-d102-763a-8807-1858b14590d2" xlink:href="prlb-20260630.xsd#prlb_EuropeSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e40b9-d102-7565-8361-1bbc3c19656e" xlink:to="loc_prlb_EuropeSegmentMember_019e40b9-d102-763a-8807-1858b14590d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_019e40b9-d102-7189-bb10-f6f11571b82a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7d2c-82ca-054168ff7b8e" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_019e40b9-d102-7189-bb10-f6f11571b82a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.protolabs.com/role/SegmentReportingScheduleofDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="prlb-20260630.xsd#SegmentReportingScheduleofDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://www.protolabs.com/role/SegmentReportingScheduleofDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7d2f-9150-c58a550f7107" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7938-a193-6d82b3dd0656" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7d2f-9150-c58a550f7107" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7938-a193-6d82b3dd0656" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e40b9-d102-75af-ba93-84735312e946" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e40b9-d102-7938-a193-6d82b3dd0656" xlink:to="loc_srt_ProductOrServiceAxis_019e40b9-d102-75af-ba93-84735312e946" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75af-ba93-84735312e946_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e40b9-d102-75af-ba93-84735312e946" xlink:to="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75af-ba93-84735312e946_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75bf-a7c2-26598aa8a469" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e40b9-d102-75af-ba93-84735312e946" xlink:to="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75bf-a7c2-26598aa8a469" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_InjectionMoldingMember_019e40b9-d102-725c-ac9e-86bc2a5268fe" xlink:href="prlb-20260630.xsd#prlb_InjectionMoldingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75bf-a7c2-26598aa8a469" xlink:to="loc_prlb_InjectionMoldingMember_019e40b9-d102-725c-ac9e-86bc2a5268fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_CNCMachiningMember_019e40b9-d102-74c3-97d6-1489441faf61" xlink:href="prlb-20260630.xsd#prlb_CNCMachiningMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75bf-a7c2-26598aa8a469" xlink:to="loc_prlb_CNCMachiningMember_019e40b9-d102-74c3-97d6-1489441faf61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_A3DPrintingMember_019e40b9-d102-729e-888f-630311cb94ce" xlink:href="prlb-20260630.xsd#prlb_A3DPrintingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75bf-a7c2-26598aa8a469" xlink:to="loc_prlb_A3DPrintingMember_019e40b9-d102-729e-888f-630311cb94ce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_SheetMetalMember_019e40b9-d102-7bf7-a6da-de10b058893a" xlink:href="prlb-20260630.xsd#prlb_SheetMetalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75bf-a7c2-26598aa8a469" xlink:to="loc_prlb_SheetMetalMember_019e40b9-d102-7bf7-a6da-de10b058893a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_OtherProductsMember_019e40b9-d102-7ed2-9571-4c0099b85ce0" xlink:href="prlb-20260630.xsd#prlb_OtherProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e40b9-d102-75bf-a7c2-26598aa8a469" xlink:to="loc_prlb_OtherProductsMember_019e40b9-d102-7ed2-9571-4c0099b85ce0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019e40b9-d102-76a3-be8b-8966e24701f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e40b9-d102-7d2f-9150-c58a550f7107" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019e40b9-d102-76a3-be8b-8966e24701f7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_prlb_DanielSchumacherMember_019f0206-c21c-7b1d-bde5-b4d93adf4179" xlink:href="prlb-20260630.xsd#prlb_DanielSchumacherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_prlb_DanielSchumacherMember_019f0206-c21c-7b1d-bde5-b4d93adf4179" xlink:type="arc" order="2"/>
  </link:definitionLink>
</link:linkbase>
