<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-02-24</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001651561</issuerCik>
        <issuerName>Tabula Rasa HealthCare, Inc.</issuerName>
        <issuerTradingSymbol>TRHC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001685832</rptOwnerCik>
            <rptOwnerName>Knowlton Orsula V</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>228 STRAWBRIDGE DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 100</rptOwnerStreet2>
            <rptOwnerCity>MOORESTOWN</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>08057</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-02-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>36710</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>767701</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-02-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>58737</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>807669</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by spouse.</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by trust for children.</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by trust for children.</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Dr. Orsula Knowlton received a restricted stock award of 36,710 shares of the Company's common stock on February 24, 2020. Such common stock will vest in four equal annual installments on January 11, 2021, January 11, 2022, January 11, 2023 and January 11, 2024.</footnote>
        <footnote id="F2">Dr. Calvin Knowlton received a restricted stock award of 58,737 shares of the Company's common stock on February 24, 2020. Such common stock will vest in four equal annual installments on January 11, 2021, January 11, 2022, January 11, 2023 and January 11, 2024.</footnote>
        <footnote id="F3">The reporting person received a restricted stock award of 267,268 shares of the Company's common stock on September 28, 2016. Pursuant to amendments to the restricted stock award on June 12, 2017, May 30, 2018, and May 2, 2019, respectively, all unvested shares of such common stock were scheduled to vest in six equal installments at the end of each calendar quarter beginning on June 30, 2019 and ending on September 30, 2020, such that such common stock would have vested in full on September 30, 2020. Notwithstanding the foregoing, on February 24, 2020, the Company entered into an amendment to the restricted stock award with the reporting person and, pursuant thereto, all shares of common stock underlying such restricted stock award will now vest on December 1, 2021.</footnote>
        <footnote id="F4">Dr. Calvin Knowlton received a restricted stock award of 337,307 shares of the Company's common stock on September 28, 2016. Pursuant to amendments to the restricted stock award on June 12, 2017, May 30, 2018, and May 2, 2019, respectively, all unvested shares of such common stock were scheduled to vest in six equal installments at the end of each calendar quarter beginning on June 30, 2019 and ending on September 30, 2020, such that such common stock would have vested in full on September 30, 2020. Notwithstanding the foregoing, on February 24, 2020, the Company entered into an amendment to the restricted stock award with Dr. Calvin Knowlton and, pursuant thereto, all shares of common stock underlying such restricted stock award will now vest on December 1, 2021.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Brian W. Adams, by Power of Attorney</signatureName>
        <signatureDate>2020-02-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
