<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-03-15T11:30:42.8545810+00:00 -->
<!-- Version            : 5.0.1.571 -->
<!-- Package ID         : 743d3140183448b4ae5e70f5ade52c9b -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
<xsd:schema xmlns:nonnum="http://www.xbrl.org/dtr/type/non-numeric" xmlns:dei="http://xbrl.sec.gov/dei/2020-01-31" xmlns:num="http://www.xbrl.org/dtr/type/numeric" xmlns:us-types="http://fasb.org/us-types/2020-01-31" xmlns:stpr="http://xbrl.sec.gov/stpr/2018-01-31" xmlns:srt-types="http://fasb.org/srt-types/2020-01-31" xmlns:sic="http://xbrl.sec.gov/sic/2020-01-31" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:srt="http://fasb.org/srt/2020-01-31" xmlns:us-gaap="http://fasb.org/us-gaap/2020-01-31" xmlns:naics="http://xbrl.sec.gov/naics/2017-01-31" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:country="http://xbrl.sec.gov/country/2020-01-31" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:currency="http://xbrl.sec.gov/currency/2020-01-31" xmlns:exch="http://xbrl.sec.gov/exch/2020-01-31" xmlns:plrx="http://www.pliantrx.com/20201231" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.pliantrx.com/20201231" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <xsd:import schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" namespace="http://www.xbrl.org/2003/instance" />
  <xsd:import schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" namespace="http://www.xbrl.org/2003/linkbase" />
  <xsd:import schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" namespace="http://xbrl.org/2005/xbrldt" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd" namespace="http://xbrl.sec.gov/country/2020-01-31" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/currency/2020/currency-2020-01-31.xsd" namespace="http://xbrl.sec.gov/currency/2020-01-31" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd" namespace="http://xbrl.sec.gov/dei/2020-01-31" />
  <xsd:import schemaLocation="http://www.xbrl.org/dtr/type/nonNumeric-2009-12-16.xsd" namespace="http://www.xbrl.org/dtr/type/non-numeric" />
  <xsd:import schemaLocation="http://www.xbrl.org/dtr/type/numeric-2009-12-16.xsd" namespace="http://www.xbrl.org/dtr/type/numeric" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/exch/2020/exch-2020-01-31.xsd" namespace="http://xbrl.sec.gov/exch/2020-01-31" />
  <xsd:import schemaLocation="http://www.xbrl.org/lrr/arcrole/factExplanatory-2009-12-16.xsd" namespace="http://www.xbrl.org/2009/arcrole/fact-explanatoryFact" />
  <xsd:import schemaLocation="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd" namespace="http://www.xbrl.org/2009/role/negated" />
  <xsd:import schemaLocation="http://www.xbrl.org/lrr/role/net-2009-12-16.xsd" namespace="http://www.xbrl.org/2009/role/net" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/naics/2017/naics-2017-01-31.xsd" namespace="http://xbrl.sec.gov/naics/2017-01-31" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/sic/2020/sic-2020-01-31.xsd" namespace="http://xbrl.sec.gov/sic/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd" namespace="http://fasb.org/srt/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/srt/2020/elts/srt-roles-2020-01-31.xsd" namespace="http://fasb.org/srt-roles/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/srt/2020/elts/srt-types-2020-01-31.xsd" namespace="http://fasb.org/srt-types/2020-01-31" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/stpr/2018/stpr-2018-01-31.xsd" namespace="http://xbrl.sec.gov/stpr/2018-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd" namespace="http://fasb.org/us-gaap/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/us-gaap/2020/elts/us-roles-2020-01-31.xsd" namespace="http://fasb.org/us-roles/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/us-gaap/2020/elts/us-types-2020-01-31.xsd" namespace="http://fasb.org/us-types/2020-01-31" />
  <xsd:annotation>
    <xsd:appinfo>
      <link:roleType id="TemplateLink" roleURI="http://www.pliantrx.com/20201231/role/TemplateLink" xmlns:link="http://www.xbrl.org/2003/linkbase">
        <link:definition>00000 - Document - Template Link</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="plrx-20201231_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:title="Calculation Links, all" xlink:type="simple" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="plrx-20201231_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:title="Definition Links, all" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="plrx-20201231_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:title="Label Links, all" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="plrx-20201231_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:title="Presentation Links, all" xlink:type="simple" />
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DocumentDocumentAndEntityInformation" id="Role_DocumentDocumentAndEntityInformation">
        <link:definition>100000 - Document - Document and Entity Information</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/StatementBalanceSheets" id="Role_StatementBalanceSheets">
        <link:definition>100010 - Statement - Balance Sheets</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/StatementBalanceSheetsParenthetical" id="Role_StatementBalanceSheetsParenthetical">
        <link:definition>100020 - Statement - Balance Sheets (Parenthetical)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/StatementStatementsOfOperationsAndComprehensiveLoss" id="Role_StatementStatementsOfOperationsAndComprehensiveLoss">
        <link:definition>100030 - Statement - Statements of Operations and Comprehensive Loss</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/StatementStatementsOfRedeemableConvertiblePreferredStockAndStockholdersEquityDeficit" id="Role_StatementStatementsOfRedeemableConvertiblePreferredStockAndStockholdersEquityDeficit">
        <link:definition>100040 - Statement - Statements of Redeemable Convertible Preferred Stock and Stockholders&apos; Equity (Deficit)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/StatementStatementsOfCashFlows" id="Role_StatementStatementsOfCashFlows">
        <link:definition>100050 - Statement - Statements of Cash Flows</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureDescriptionOfBusiness" id="Role_DisclosureDescriptionOfBusiness">
        <link:definition>100060 - Disclosure - Description of Business</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies" id="Role_DisclosureSummaryOfSignificantAccountingPolicies">
        <link:definition>100070 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureFinancialInstruments" id="Role_DisclosureFinancialInstruments">
        <link:definition>100080 - Disclosure - Financial Instruments</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosurePropertyAndEquipmentNet" id="Role_DisclosurePropertyAndEquipmentNet">
        <link:definition>100090 - Disclosure - Property and Equipment, net</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureAccruedLiabilitiesAndOtherLongTermLiabilities" id="Role_DisclosureAccruedLiabilitiesAndOtherLongTermLiabilities">
        <link:definition>100100 - Disclosure - Accrued Liabilities and Other Long-Term Liabilities</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureNovartisAgreement" id="Role_DisclosureNovartisAgreement">
        <link:definition>100110 - Disclosure - Novartis Agreement</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureLicenseAgreements" id="Role_DisclosureLicenseAgreements">
        <link:definition>100120 - Disclosure - License Agreements</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureResearchAgreement" id="Role_DisclosureResearchAgreement">
        <link:definition>100130 - Disclosure - Research Agreement</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStock" id="Role_DisclosureRedeemableConvertiblePreferredStock">
        <link:definition>100140 - Disclosure - Redeemable Convertible Preferred Stock</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureCommonStock" id="Role_DisclosureCommonStock">
        <link:definition>100150 - Disclosure - Common Stock</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensation" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensation">
        <link:definition>100160 - Disclosure - Equity Incentive Plans and Stock-Based Compensation</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureIncomeTaxes" id="Role_DisclosureIncomeTaxes">
        <link:definition>100170 - Disclosure - Income Taxes</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureCommitmentsAndContingencies" id="Role_DisclosureCommitmentsAndContingencies">
        <link:definition>100180 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureRelatedPartyTransactions" id="Role_DisclosureRelatedPartyTransactions">
        <link:definition>100190 - Disclosure - Related Party Transactions</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureDefinedContributionPlan" id="Role_DisclosureDefinedContributionPlan">
        <link:definition>100200 - Disclosure - Defined Contribution Plan</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonStockholders" id="Role_DisclosureNetLossPerShareAttributableToCommonStockholders">
        <link:definition>100210 - Disclosure - Net Loss Per Share Attributable to Common Stockholders</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies" id="Role_DisclosureSummaryOfSignificantAccountingPoliciesPolicies">
        <link:definition>100220 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesTables" id="Role_DisclosureSummaryOfSignificantAccountingPoliciesTables">
        <link:definition>100230 - Disclosure - Summary of Significant Accounting Policies (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureFinancialInstrumentsTables" id="Role_DisclosureFinancialInstrumentsTables">
        <link:definition>100240 - Disclosure - Financial Instruments (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosurePropertyAndEquipmentNetTables" id="Role_DisclosurePropertyAndEquipmentNetTables">
        <link:definition>100250 - Disclosure - Property and Equipment, net (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureAccruedLiabilitiesAndOtherLongTermLiabilitiesTables" id="Role_DisclosureAccruedLiabilitiesAndOtherLongTermLiabilitiesTables">
        <link:definition>100260 - Disclosure - Accrued Liabilities and Other Long-Term Liabilities (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStockTables" id="Role_DisclosureRedeemableConvertiblePreferredStockTables">
        <link:definition>100270 - Disclosure - Redeemable Convertible Preferred Stock (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureCommonStockTables" id="Role_DisclosureCommonStockTables">
        <link:definition>100280 - Disclosure - Common Stock (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationTables" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensationTables">
        <link:definition>100290 - Disclosure - Equity Incentive Plans and Stock-Based Compensation (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureIncomeTaxesTables" id="Role_DisclosureIncomeTaxesTables">
        <link:definition>100300 - Disclosure - Income Taxes (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureCommitmentsAndContingenciesTables" id="Role_DisclosureCommitmentsAndContingenciesTables">
        <link:definition>100310 - Disclosure - Commitments and Contingencies (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonStockholdersTables" id="Role_DisclosureNetLossPerShareAttributableToCommonStockholdersTables">
        <link:definition>100320 - Disclosure - Net Loss Per Share Attributable to Common Stockholders (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureDescriptionOfBusinessAdditionalInformationDetail" id="Role_DisclosureDescriptionOfBusinessAdditionalInformationDetail">
        <link:definition>100330 - Disclosure - Description of Business - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetails" id="Role_DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetails">
        <link:definition>100340 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesSummaryOfUsefulLivesOfPropertyAndEquipmentDetails" id="Role_DisclosureSummaryOfSignificantAccountingPoliciesSummaryOfUsefulLivesOfPropertyAndEquipmentDetails">
        <link:definition>100350 - Disclosure - Summary of Significant Accounting Policies - Summary of Useful Lives of Property and Equipment (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureFinancialInstrumentsSummaryOfCashAndCashEquivalentsMoneyMarketFundsAndShortTermInvestmentsBySignificantInvestmentCategoryDetails" id="Role_DisclosureFinancialInstrumentsSummaryOfCashAndCashEquivalentsMoneyMarketFundsAndShortTermInvestmentsBySignificantInvestmentCategoryDetails">
        <link:definition>100360 - Disclosure - Financial Instruments - Summary of Cash and Cash Equivalents, Money Market Funds and Short-term Investments by Significant Investment Category (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureFinancialInstrumentsSummeryOfRemainingContractualMaturitiesOfCompanySShortTermInvestmentsDetails" id="Role_DisclosureFinancialInstrumentsSummeryOfRemainingContractualMaturitiesOfCompanySShortTermInvestmentsDetails">
        <link:definition>100370 - Disclosure - Financial Instruments - Summery of Remaining Contractual Maturities of Company&apos;s Short-term Investments (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureFinancialInstrumentsAdditionalInformationDetails" id="Role_DisclosureFinancialInstrumentsAdditionalInformationDetails">
        <link:definition>100380 - Disclosure - Financial Instruments - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentDetails" id="Role_DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentDetails">
        <link:definition>100390 - Disclosure - Property and Equipment, net - Schedule of Property and Equipment (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosurePropertyAndEquipmentNetAdditionalInformationDetails" id="Role_DisclosurePropertyAndEquipmentNetAdditionalInformationDetails">
        <link:definition>100400 - Disclosure - Property and Equipment, net - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureAccruedLiabilitiesAndOtherLongTermLiabilitiesSummaryOfAccruedLiabilitiesDetails" id="Role_DisclosureAccruedLiabilitiesAndOtherLongTermLiabilitiesSummaryOfAccruedLiabilitiesDetails">
        <link:definition>100410 - Disclosure - Accrued Liabilities and Other Long-Term Liabilities - Summary of Accrued Liabilities (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureAccruedLiabilitiesAndOtherLongTermLiabilitiesSummaryOfOtherLongTermLiabilitiesDetails" id="Role_DisclosureAccruedLiabilitiesAndOtherLongTermLiabilitiesSummaryOfOtherLongTermLiabilitiesDetails">
        <link:definition>100420 - Disclosure - Accrued Liabilities and Other Long-Term Liabilities - Summary of Other Long-Term Liabilities (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureNovartisAgreementAdditionalInformationDetails" id="Role_DisclosureNovartisAgreementAdditionalInformationDetails">
        <link:definition>100430 - Disclosure - Novartis Agreement - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureLicenseAgreementsAdditionalInformationDetails" id="Role_DisclosureLicenseAgreementsAdditionalInformationDetails">
        <link:definition>100440 - Disclosure - License Agreements - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureResearchAgreementAdditionalInformationDetails" id="Role_DisclosureResearchAgreementAdditionalInformationDetails">
        <link:definition>100450 - Disclosure - Research Agreement - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStockAdditionalInformationDetails" id="Role_DisclosureRedeemableConvertiblePreferredStockAdditionalInformationDetails">
        <link:definition>100460 - Disclosure - Redeemable Convertible Preferred Stock - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStockSummaryOfRedeemableConvertiblePreferredStockDetails" id="Role_DisclosureRedeemableConvertiblePreferredStockSummaryOfRedeemableConvertiblePreferredStockDetails">
        <link:definition>100470 - Disclosure - Redeemable Convertible Preferred Stock - Summary of Redeemable Convertible Preferred Stock (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureCommonStockAdditionalInformationDetails" id="Role_DisclosureCommonStockAdditionalInformationDetails">
        <link:definition>100480 - Disclosure - Common Stock - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureCommonStockScheduleOfSharesReservedForFutureIssuanceDetails" id="Role_DisclosureCommonStockScheduleOfSharesReservedForFutureIssuanceDetails">
        <link:definition>100490 - Disclosure - Common Stock - Schedule of Shares Reserved for Future Issuance (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationAdditionalInformationDetails" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensationAdditionalInformationDetails">
        <link:definition>100500 - Disclosure - Equity Incentive Plans and Stock-Based Compensation - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfRestrictedStockActivityDetails" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfRestrictedStockActivityDetails">
        <link:definition>100510 - Disclosure - Equity Incentive Plans and Stock-Based Compensation - Summary of Restricted Stock Activity (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfBlackScholesOptionPricingModelToEstimateStockBasedCompensationExpenseForStockOptionAwardsDetails" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfBlackScholesOptionPricingModelToEstimateStockBasedCompensationExpenseForStockOptionAwardsDetails">
        <link:definition>100520 - Disclosure - Equity Incentive Plans and Stock-Based Compensation - Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for Stock Option Awards (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfOptionActivityUnder2015PlanAnd2020PlanDetails" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfOptionActivityUnder2015PlanAnd2020PlanDetails">
        <link:definition>100530 - Disclosure - Equity Incentive Plans and Stock-Based Compensation - Summary of Option Activity Under the 2015 Plan and 2020 Plan (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationIncentiveStockOptionsAndNonqualifiedStockOptionsAdditionalInformationDetails" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensationIncentiveStockOptionsAndNonqualifiedStockOptionsAdditionalInformationDetails">
        <link:definition>100540 - Disclosure - Equity Incentive Plans and Stock-Based Compensation - Incentive Stock Options and Nonqualified Stock Options - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensation2020EmployeeStockPurchasePlanAdditionalInformationDetails" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensation2020EmployeeStockPurchasePlanAdditionalInformationDetails">
        <link:definition>100550 - Disclosure - Equity Incentive Plans and Stock-Based Compensation - 2020 Employee Stock Purchase Plan - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensation2020EmployeeStockPurchasePlanSummaryOfBlackScholesOptionPricingModelToEstimateStockBasedCompensationExpenseFor2020EmployeeStockPurchasePlanDetails" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensation2020EmployeeStockPurchasePlanSummaryOfBlackScholesOptionPricingModelToEstimateStockBasedCompensationExpenseFor2020EmployeeStockPurchasePlanDetails">
        <link:definition>100560 - Disclosure - Equity Incentive Plans and Stock-Based Compensation - 2020 Employee Stock Purchase Plan - Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for 2020 Employee Stock Purchase Plan (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfClassificationOfStockBasedCompensationExpenseDetails" id="Role_DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfClassificationOfStockBasedCompensationExpenseDetails">
        <link:definition>100570 - Disclosure - Equity Incentive Plans and Stock-Based Compensation - Summary of Classification of Stock-based Compensation Expense (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetails" id="Role_DisclosureIncomeTaxesAdditionalInformationDetails">
        <link:definition>100580 - Disclosure - Income Taxes - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureIncomeTaxesSummaryOfReconciliationOfUSFederalStatutoryIncomeTaxRateToCompanySEffectiveIncomeTaxRateDetails" id="Role_DisclosureIncomeTaxesSummaryOfReconciliationOfUSFederalStatutoryIncomeTaxRateToCompanySEffectiveIncomeTaxRateDetails">
        <link:definition>100590 - Disclosure - Income Taxes - Summary of Reconciliation of U.S. Federal Statutory Income Tax Rate to Company&#8217;s Effective Income Tax Rate (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureIncomeTaxesSummaryOfDeferredTaxAssetsAndLiabilitiesDetails" id="Role_DisclosureIncomeTaxesSummaryOfDeferredTaxAssetsAndLiabilitiesDetails">
        <link:definition>100600 - Disclosure - Income Taxes - Summary of Deferred Tax Assets and Liabilities (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureIncomeTaxesSummaryOfNetOperatingLossesAndTaxCreditCarryForwardsDetails" id="Role_DisclosureIncomeTaxesSummaryOfNetOperatingLossesAndTaxCreditCarryForwardsDetails">
        <link:definition>100610 - Disclosure - Income Taxes - Summary of Net Operating Losses and Tax Credit Carry Forwards (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureIncomeTaxesSummaryOfChangesInValuationAllowanceForDeferredTaxAssetsDetails" id="Role_DisclosureIncomeTaxesSummaryOfChangesInValuationAllowanceForDeferredTaxAssetsDetails">
        <link:definition>100620 - Disclosure - Income Taxes - Summary of Changes in Valuation Allowance for Deferred Tax Assets (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureIncomeTaxesSummaryOfReconciliationOfUnrecognizedTaxBenefitsDetails" id="Role_DisclosureIncomeTaxesSummaryOfReconciliationOfUnrecognizedTaxBenefitsDetails">
        <link:definition>100630 - Disclosure - Income Taxes - Summary of Reconciliation of Unrecognized Tax Benefits (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails" id="Role_DisclosureCommitmentsAndContingenciesAdditionalInformationDetails">
        <link:definition>100640 - Disclosure - Commitments and Contingencies - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureCommitmentsAndContingenciesScheduleOfFutureMinimumLeasePaymentsDetails" id="Role_DisclosureCommitmentsAndContingenciesScheduleOfFutureMinimumLeasePaymentsDetails">
        <link:definition>100650 - Disclosure - Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetails" id="Role_DisclosureRelatedPartyTransactionsAdditionalInformationDetails">
        <link:definition>100660 - Disclosure - Related Party Transactions - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureDefinedContributionPlanAdditionalInformationDetails" id="Role_DisclosureDefinedContributionPlanAdditionalInformationDetails">
        <link:definition>100670 - Disclosure - Defined Contribution Plan - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonStockholdersSummaryOfPotentiallyDilutiveSecuritiesExcludedFromComputationOfDilutedNetLossPerShareDetails" id="Role_DisclosureNetLossPerShareAttributableToCommonStockholdersSummaryOfPotentiallyDilutiveSecuritiesExcludedFromComputationOfDilutedNetLossPerShareDetails">
        <link:definition>100680 - Disclosure - Net Loss Per Share Attributable to Common Stockholders - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.pliantrx.com/20201231/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonStockholdersReconciliationOfNumeratorAndDenominatorUsedInCalculationOfBasicAndDilutedNetLossPerShareDetails" id="Role_DisclosureNetLossPerShareAttributableToCommonStockholdersReconciliationOfNumeratorAndDenominatorUsedInCalculationOfBasicAndDilutedNetLossPerShareDetails">
        <link:definition>100690 - Disclosure - Net Loss Per Share Attributable to Common Stockholders - Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xsd:appinfo>
  </xsd:annotation>
  <xsd:element id="plrx_AccretionToRedemptionValueAndDividendsOnRedeemableConvertiblePreferredStock" name="AccretionToRedemptionValueAndDividendsOnRedeemableConvertiblePreferredStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_SeriesARedeemableConvertiblePreferredStockMember" name="SeriesARedeemableConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_SeriesBRedeemableConvertiblePreferredStockMember" name="SeriesBRedeemableConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_SeriesCRedeemableConvertiblePreferredStockMember" name="SeriesCRedeemableConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues" name="TemporaryEquityStockIssuedDuringPeriodSharesNewIssues" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_TemporaryEquityStockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" name="TemporaryEquityStockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_TemporaryEquityStockIssuedDuringPeriodValueConversionOfConvertibleSecurities" name="TemporaryEquityStockIssuedDuringPeriodValueConversionOfConvertibleSecurities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_AccretionToRedemptionValueAndCumulativeDividendOnRedeemableConvertiblePreferredStock" name="AccretionToRedemptionValueAndCumulativeDividendOnRedeemableConvertiblePreferredStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_OtherLosses" name="OtherLosses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_AccretionOfShortTermInvestments" name="AccretionOfShortTermInvestments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_ReclassificationOfRestrictedStockAwardsFromLiabilitiesToCommonStockUponVesting" name="ReclassificationOfRestrictedStockAwardsFromLiabilitiesToCommonStockUponVesting" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_DeferredOfferingCostsInAccountsPayableAndAccruedLiabilities" name="DeferredOfferingCostsInAccountsPayableAndAccruedLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_LicenseAgreementsTextBlock" name="LicenseAgreementsTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_LicenseAgreementsAbstract" name="LicenseAgreementsAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_RedeemableConvertiblePreferredStockTextBlock" name="RedeemableConvertiblePreferredStockTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_RedeemableConvertiblePreferredStockPolicyPolicyTextBlock" name="RedeemableConvertiblePreferredStockPolicyPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_DeferredOfferingCostsPolicyTextBlock" name="DeferredOfferingCostsPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_PropertyPlantAndEquipmentUsefulLivesTableTextBlock" name="PropertyPlantAndEquipmentUsefulLivesTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ScheduleOfCommonStockReservedForFutureIssuanceTableTextBlock" name="ScheduleOfCommonStockReservedForFutureIssuanceTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ScheduleOfNetOperatingLossesAndTaxCreditCarryforwardsTableTextBlock" name="ScheduleOfNetOperatingLossesAndTaxCreditCarryforwardsTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_UnderwritersMember" name="UnderwritersMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_PliantTherapeuticsIncMember" name="PliantTherapeuticsIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_NovartisMember" name="NovartisMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_AccountingPoliciesLineItems" name="AccountingPoliciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_AccountingPoliciesTable" name="AccountingPoliciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_NovartisAgreementMember" name="NovartisAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_CustomerPaymentTerms" name="CustomerPaymentTerms" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_PercentageOfRevenueContributedByRelatedParty" name="PercentageOfRevenueContributedByRelatedParty" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_DebtInstrumentRemainingMaturityPeriodAtDateOfPurchase" name="DebtInstrumentRemainingMaturityPeriodAtDateOfPurchase" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ComputerEquipmentAndSoftwareMember" name="ComputerEquipmentAndSoftwareMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_AmountOfGrossReceiptsDuringPeriod" name="AmountOfGrossReceiptsDuringPeriod" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_MatureInOneYearOrLessMember" name="MatureInOneYearOrLessMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_MatureInOneToTwoYearsMember" name="MatureInOneToTwoYearsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_AccruedResearchAndDevelopmentExpenses" name="AccruedResearchAndDevelopmentExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_DeferredRentAndLeaseholdIncentiveObligationCurrent" name="DeferredRentAndLeaseholdIncentiveObligationCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_ProceedsFromUpfrontNonRefundableLicenseFee" name="ProceedsFromUpfrontNonRefundableLicenseFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_NumberOfIntegrinResearchTargets" name="NumberOfIntegrinResearchTargets" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_LicenseFeeMember" name="LicenseFeeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_AchievementOfFirstPatientDosingMilestoneMember" name="AchievementOfFirstPatientDosingMilestoneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_CollaborationAndLicenseAgreementTargetValidationFee" name="CollaborationAndLicenseAgreementTargetValidationFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_CollaborationAndLicenseAgreementIntegrinResearchTargetsAmount" name="CollaborationAndLicenseAgreementIntegrinResearchTargetsAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_RedeemableConvertiblePreferredStockIssuedAtFairValuePerShare" name="RedeemableConvertiblePreferredStockIssuedAtFairValuePerShare" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="plrx_PercentageOfOutstandingSharesOfCommonStockOnFullyDilutedBasis" name="PercentageOfOutstandingSharesOfCommonStockOnFullyDilutedBasis" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="plrx_SellingPriceOfLicense" name="SellingPriceOfLicense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_PaymentOfNonRefundableUpfrontLicenseFee" name="PaymentOfNonRefundableUpfrontLicenseFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_PaymentOfAnnualLicenseMaintenanceFee" name="PaymentOfAnnualLicenseMaintenanceFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_PaymentOfAnnualLicenseMaintenanceFeeAfterFirstYear" name="PaymentOfAnnualLicenseMaintenanceFeeAfterFirstYear" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_PercentageOfRoyaltyPaymentsOnNetSalesOfLicensedProduct" name="PercentageOfRoyaltyPaymentsOnNetSalesOfLicensedProduct" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_PercentageOfRoyaltyPaymentsOnNetSalesOfMethodUseLicensedProduct" name="PercentageOfRoyaltyPaymentsOnNetSalesOfMethodUseLicensedProduct" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_AnnualMinimumRoyaltyPaymentOnNetSales" name="AnnualMinimumRoyaltyPaymentOnNetSales" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_AggregateContingencyPaymentOnCertainEventsMilestones" name="AggregateContingencyPaymentOnCertainEventsMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_MilestoneAndAnnualRoyaltyPaid" name="MilestoneAndAnnualRoyaltyPaid" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_LicenseAgreementsLineItems" name="LicenseAgreementsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_LicenseAgreementsTable" name="LicenseAgreementsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_UCRegentsMember" name="UCRegentsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_DevelopmentAndOptionAgreementDate" name="DevelopmentAndOptionAgreementDate" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_AdimabAgreementMember" name="AdimabAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_FullTimeEmployeeCostMember" name="FullTimeEmployeeCostMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_AntibodyDiscoveryFeesMember" name="AntibodyDiscoveryFeesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_SeriesAPurchaseAgreementMember" name="SeriesAPurchaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_SeriesBPurchaseAgreementMember" name="SeriesBPurchaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_SeriesCPurchaseAgreementMember" name="SeriesCPurchaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_SeriesASeriesBAndSeriesCPreferredStockMember" name="SeriesASeriesBAndSeriesCPreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_AmendedAndRestatedCertificateOfIncorporationMember" name="AmendedAndRestatedCertificateOfIncorporationMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_TemporaryEquityAuthorizedValue" name="TemporaryEquityAuthorizedValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_NumberOfTranches" name="NumberOfTranches" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_RedeemableConvertiblePreferredStock" name="RedeemableConvertiblePreferredStock" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_TemporaryEquityConversionPricePerShare" name="TemporaryEquityConversionPricePerShare" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="plrx_TemporaryEquityDividendRatePercentage" name="TemporaryEquityDividendRatePercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_RedeemableConvertiblePreferredStockRedemptionTerms" name="RedeemableConvertiblePreferredStockRedemptionTerms" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_FoundersCommonStockAwardsMember" name="FoundersCommonStockAwardsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_TwoThousandFifteenEquityIncentivePlanMember" name="TwoThousandFifteenEquityIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_TwoThousandTwentyEquityIncentivePlanMember" name="TwoThousandTwentyEquityIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_TwoThousandAndTwentyEmployeeStockPurchasePlanMember" name="TwoThousandAndTwentyEmployeeStockPurchasePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_TwoThousandAndTwentyStockOptionsAndIncentivePlanMember" name="TwoThousandAndTwentyStockOptionsAndIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_RestrictedCommonStockAwardsMember" name="RestrictedCommonStockAwardsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_AccruedExpensesAndOtherLiabilitiesMember" name="AccruedExpensesAndOtherLiabilitiesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_RestrictedStockAwardsMember" name="RestrictedStockAwardsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_TwoThousandFifteenAndTwoThousandTwentyEquityIncentivePlanMember" name="TwoThousandFifteenAndTwoThousandTwentyEquityIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_ShareBasedCompensationOptionsUnderPlanCouldBeGrantedForPeriods" name="ShareBasedCompensationOptionsUnderPlanCouldBeGrantedForPeriods" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardExercisePriceWithPurchasePriceOfCommonStockPercentageOfEstimatedGrantDateFairValueForShareholders" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardExercisePriceWithPurchasePriceOfCommonStockPercentageOfEstimatedGrantDateFairValueForShareholders" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ShareBasedCompensationOptionsUnderPlanMayBeGrantedForPeriods" name="ShareBasedCompensationOptionsUnderPlanMayBeGrantedForPeriods" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRepurchasesInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRepurchasesInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRepurchasesWeightedAverageGrantDateFairValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRepurchasesWeightedAverageGrantDateFairValue" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ShareBasedCompensationNumberOfRestrictedStockAwardsNotPurchased" name="ShareBasedCompensationNumberOfRestrictedStockAwardsNotPurchased" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="plrx_ShareBasedCompensationRestrictedStockAwardsWeightedAverageGrantDateFairValueNotPurchased" name="ShareBasedCompensationRestrictedStockAwardsWeightedAverageGrantDateFairValueNotPurchased" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="plrx_UnderlyingCommonStockFairValue" name="UnderlyingCommonStockFairValue" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_IncentiveStockOptionsAndNonqualifiedStockOptionsMember" name="IncentiveStockOptionsAndNonqualifiedStockOptionsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_EmployeeStockPurchasePlanOfferingPeriod" name="EmployeeStockPurchasePlanOfferingPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_CommonStockFairMarketValuePercentage" name="CommonStockFairMarketValuePercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_ValuationAllowanceAsPercentageOnDeferredTaxAssets" name="ValuationAllowanceAsPercentageOnDeferredTaxAssets" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="plrx_EffectiveIncomeTaxRateReconciliationGeneralBusinessTaxCreditsFederal" name="EffectiveIncomeTaxRateReconciliationGeneralBusinessTaxCreditsFederal" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_DeferredTaxLiabilitiesAssetBasis" name="DeferredTaxLiabilitiesAssetBasis" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_StartingFromJanuary12018Member" name="StartingFromJanuary12018Member" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_BeforeJanuary12018Member" name="BeforeJanuary12018Member" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_OperatingLossCarryforwardsExpirationYear" name="OperatingLossCarryforwardsExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_TaxCreditCarryforwardsExpirationYear" name="TaxCreditCarryforwardsExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_IncomeTaxExaminationNumberOfExaminationsPending" name="IncomeTaxExaminationNumberOfExaminationsPending" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_UnrecognizedTaxBenefitsIncreasesDecreasesResultingFromPriorPeriodTaxPositions" name="UnrecognizedTaxBenefitsIncreasesDecreasesResultingFromPriorPeriodTaxPositions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_ContractualArrangementsCancellationPeriod" name="ContractualArrangementsCancellationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_CommitmentsLineItems" name="CommitmentsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_CommitmentsTable" name="CommitmentsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_LeasedSpaceUnderNonCancellableOperatingLease" name="LeasedSpaceUnderNonCancellableOperatingLease" type="num:areaItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_BaseRentAbatedAmount" name="BaseRentAbatedAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_LeaseTermExtensionNoticePeriod" name="LeaseTermExtensionNoticePeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="plrx_OperatingLeaseRentExpenseAndCommonAreaMaintenanceCharges" name="OperatingLeaseRentExpenseAndCommonAreaMaintenanceCharges" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="plrx_ThirdRockVenturesMember" name="ThirdRockVenturesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_NonemployeeDirectorMember" name="NonemployeeDirectorMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_UCSFFoundationMember" name="UCSFFoundationMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_ScientificFoundersMember" name="ScientificFoundersMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="plrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodFairValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodFairValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="plrx_PercentageOfOutstandingSharesOnFullyDilutedBasis" name="PercentageOfOutstandingSharesOnFullyDilutedBasis" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="plrx_RestrictedStockAwardsGrantedAndNotPurchasedMember" name="RestrictedStockAwardsGrantedAndNotPurchasedMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
