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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-09-12</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001069308</issuerCik>
        <issuerName>Opexa Therapeutics, Inc.</issuerName>
        <issuerTradingSymbol>OPXA</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001437911</rptOwnerCik>
            <rptOwnerName>Warma Neil K</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OPEXA THERAPEUTICS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2635 TECHNOLOGY FOREST BLVD.</rptOwnerStreet2>
            <rptOwnerCity>THE WOODLANDS</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77381</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President &amp; CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-12-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>600</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>1.33</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>8658</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Employee Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>3.80</value>
                <footnoteId id="F4"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-09-12</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>43623</value>
                    <footnoteId id="F4"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2012-09-12</value>
            </exerciseDate>
            <expirationDate>
                <value>2022-01-06</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>43623</value>
                    <footnoteId id="F4"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>43623</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On December 31, 2012, the reporting person was issued 600 shares of Common Stock as payment-in-kind for interest accrued as of December 31, 2012 on a 12% Convertible Secured Promissory Note held in the principal amount of $15,000. The shares issued in payment of accrued interest payable on the interest payment date were deemed to have a value equal to the volume weighted average price for the Common Stock for the five trading days ending three trading days prior to the payment date.</footnote>
        <footnote id="F2">Adjusted to reflect the issuer's 1-for-4 reverse stock split that was implemented December 14, 2012, resulting in the reporting person owning 8,058 shares of common stock prior to the transaction reported herein.</footnote>
        <footnote id="F3">On January 6, 2012, the reporting person was granted an option to purchase 139,593 shares of common stock at the closing market price.  The option is performance?based and begins to vest quarterly over a three-year period in two tranches commencing upon Opexa achieving certain key milestone events.  The first tranche key milestone for vesting of 43,623 shares is Opexa initiating a clinical trial for Tcelna in SPMS.  The second tranche key milestone for vesting of 95,970 shares is Opexa entering into a collaboration, partnership or other strategic arrangement involving US rights for Tcelna, provided that vesting for 8,724 shares in the second tranche commences three-year quarterly vesting earlier upon Opexa entering into a collaboration, partnership or other strategic arrangement involving any rights proprietary to Opexa in Asia or Europe.  The performance criteria for the first key milestone has been met and vesting has commenced for the first tranche of 43,623 shares.</footnote>
        <footnote id="F4">All share numbers and the exercise price have been adjusted to reflect the one-for-four reverse stock split implemented December 14, 2012.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Neil K. Warma</signatureName>
        <signatureDate>2013-01-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
