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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-01-29</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0000897075</issuerCik>
        <issuerName>REPROS THERAPEUTICS INC.</issuerName>
        <issuerTradingSymbol>RPRX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001274244</rptOwnerCik>
            <rptOwnerName>CAIN DANIEL F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1179 CENTENNIAL ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>FORT COLLINS</rptOwnerCity>
            <rptOwnerState>CO</rptOwnerState>
            <rptOwnerZipCode>80525</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2018-01-29</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>U</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>23221</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.67</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>27779</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2018-01-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>27779</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.67</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0.32</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2018-01-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2027-09-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Consists of restricted stock units. Pursuant to the terms of the Agreement and Plan of Merger (the &quot;Merger Agreement&quot;), dated December 11, 2018, by and among Repros Therapeutics Inc. (the &quot;Issuer&quot;), Allergan Sales, LLC (&quot;Parent&quot;) and Celestial Merger Sub, Inc. (&quot;Purchaser&quot;), pursuant to which Purchaser was merged with and into Issuer (the &quot;Merger&quot;), each outstanding restricted stock unit and any associated rights to the issuance of shares of the Issuer's common stock, par value $0.001 per share (the &quot;Shares&quot;) upon the achievement of Issuer performance goals (the &quot;Issuer RSUs&quot;) under the Issuer's equity plans that was not then vested was cancelled, (continued in Footnote 2)</footnote>
        <footnote id="F2">(continued from Footnote 1) and the holder of such cancelled Issuer RSUs became entitled to receive, in exchange therefor, an amount in cash equal to the product of (x) the total number of Shares subject to (or deliverable under) such Issuer RSUs immediately prior to the effective time of the Merger (the &quot;Effective Time&quot;) (determined after giving effect to the accelerated vesting and deemed achievement at maximum applicable performance levels) multiplied by (y) $0.67 in cash (the &quot;Closing Amount&quot;). These restricted stock units were fully vested immediately prior to the Effective Time.</footnote>
        <footnote id="F3">Pursuant to the terms of the Merger Agreement, each option to purchase Shares (a &quot;Company Option&quot;) that was then outstanding and unexercised, whether or not vested and which had a per-Share exercise price less than the Closing Amount (each, an &quot;In the Money Option&quot;), was cancelled and converted into the right to receive (i) a cash payment equal to (A) the excess, if any, of (x) the Closing Amount over (y) the exercise price payable per Share under such In the Money Option, multiplied by (B) the total number of Shares subject to such In the Money Option immediately prior to the Effective Time. At the Effective Time, each Company Option other than an In the Money Option that was then outstanding and unexercised, whether or not vested (each, an &quot;Out of the Money Option&quot;), was cancelled at the Effective Time without any consideration payable therefor.</footnote>
        <footnote id="F4">The Shares subject to this option were fully vested immediately prior to the Effective Time.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Katherine A. Anderson, Attorney-in-fact</signatureName>
        <signatureDate>2018-01-31</signatureDate>
    </ownerSignature>
</ownershipDocument>
