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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-03-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001379895</issuerCik>
        <issuerName>DYNEGY INC.</issuerName>
        <issuerTradingSymbol>DYN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000921669</rptOwnerCik>
            <rptOwnerName>ICAHN CARL C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ICAHN ASSOCIATES CORP.</rptOwnerStreet1>
            <rptOwnerStreet2>767 FIFTH AVE., SUITE 4700</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10153</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001412091</rptOwnerCik>
            <rptOwnerName>IPH GP LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>445 HAMILTON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1210</rptOwnerStreet2>
            <rptOwnerCity>WHITE PLAINS</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001412093</rptOwnerCik>
            <rptOwnerName>ICAHN CAPITAL LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>445 HAMILTON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1210</rptOwnerStreet2>
            <rptOwnerCity>WHITE PLAINS</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001257324</rptOwnerCik>
            <rptOwnerName>ICAHN ENTERPRISES G.P. INC.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>445 HAMILTON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1210</rptOwnerStreet2>
            <rptOwnerCity>WHITE PLAINS</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001034563</rptOwnerCik>
            <rptOwnerName>ICAHN ENTERPRISES HOLDINGS L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>445 HAMILTON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1210</rptOwnerStreet2>
            <rptOwnerCity>WHITE PLAINS</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001080113</rptOwnerCik>
            <rptOwnerName>BECKTON CORP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>445 HAMILTON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1210</rptOwnerStreet2>
            <rptOwnerCity>WHITE PLAINS</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share (&quot;Common Shares&quot;)</value>
            </securityTitle>
            <transactionDate>
                <value>2011-03-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>X</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6042212</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F9"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18042212</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>please see footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Call option (&quot;right to purchase&quot;)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.90</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2011-03-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>X</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6042212</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>1.69</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F8"/>
            </exerciseDate>
            <expirationDate>
                <value>2012-11-10</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6042212</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>please see footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Put option (&quot;obligation to purchase&quot;)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.9</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2011-03-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>X</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6042212</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.01</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F8"/>
            </exerciseDate>
            <expirationDate>
                <value>2012-11-10</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6042212</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>please see all footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">High River Limited Partnership (&quot;High River&quot;) directly beneficially owns 3,608,441 Shares, Icahn Partners LP (&quot;Icahn Partners&quot;) directly beneficially owns 5,510,709 Shares, Icahn Partners Master Fund LP (&quot;Icahn Master&quot;) directly beneficially owns 6,238,314 Shares, Icahn Partners Master Fund II LP (&quot;Icahn Master II&quot;) directly beneficially owns 1,805,714 Shares, and Icahn Partners Master Fund III LP (&quot;Icahn Master III&quot;) directly beneficially owns 879,034 Shares.</footnote>
        <footnote id="F2">Barberry Corp. (&quot;Barberry&quot;), is the sole member of Hopper Investments LLC (&quot;Hopper&quot;), which is the general partner of High River. Beckton Corp. (&quot;Beckton&quot;) is the sole stockholder of Icahn Enterprises G.P. Inc. (&quot;Icahn Enterprises GP&quot;), which is the general partner of Icahn Enterprises Holdings L.P. (&quot;Icahn Enterprises Holdings&quot;). Icahn Enterprises Holdings is the sole member of IPH GP LLC (&quot;IPH&quot;), which is the general partner of Icahn Capital LP (&quot;Icahn Capital&quot;). Icahn Capital is the general partner of each of Icahn Onshore LP (&quot;Icahn Onshore&quot;) and Icahn Offshore LP (&quot;Icahn Offshore&quot;). Icahn Onshore is the general partner of Icahn Partners. Icahn Offshore is the general partner of each of Icahn Master, Icahn Master II and Icahn Master III.</footnote>
        <footnote id="F3">Each of Barberry and Beckton is 100 percent owned by Carl C. Icahn. As such, Mr. Icahn is in a position indirectly to determine the investment and voting decisions made by each of High River, Icahn Partners, Icahn Master, Icahn Master II and Icahn Master III. Each of Hopper, Barberry and Mr. Icahn may be deemed to indirectly beneficially own (as that term is defined in Rule 13d-3 under the Act) the Shares which High River owns. Each of Hopper, Barberry and Mr. Icahn disclaims beneficial ownership of such Shares except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F4">Each of Icahn Onshore, Icahn Capital, IPH, Icahn Enterprises Holdings, Icahn Enterprises GP, Beckton and Mr. Icahn may be deemed to indirectly beneficially own (as that term is defined in Rule 13d-3 under the Act) the Shares which Icahn Partners owns. Each of Icahn Onshore, Icahn Capital, IPH, Icahn Enterprises Holdings, Icahn Enterprises GP, Beckton and Mr. Icahn disclaims beneficial ownership of such Shares except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F5">Each of Icahn Offshore, Icahn Capital, IPH, Icahn Enterprises Holdings, Icahn Enterprises GP, Beckton and Mr. Icahn may be deemed to indirectly beneficially own (as that term is defined in Rule 13d-3 under the Act) the Shares which each of Icahn Master, Icahn Master II and Icahn Master III owns. Each of Icahn Offshore, Icahn Capital, IPH, Icahn Enterprises Holdings, Icahn Enterprises GP, Beckton and Mr. Icahn disclaims beneficial ownership of such Shares except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F6">High River directly beneficially owned 1,208,442 call options (the &quot;Call Options&quot;), Icahn Partners directly beneficially owned 1,857,034 Call Options, Icahn Master directly beneficially owned 2,092,971 Call Options, Icahn Master II directly beneficially owned 590,137 Call Options, and Icahn Master III directly beneficially owned 293,628 Call Options.   On March 1, 2011, each of High River, Icahn Partners, Icahn Master, Icahn Master II and Icahn Master III exercised all of their respective Call Options in their entirety, and thereby acquired 1,208,442 Shares, 1,857,034 Shares, 2,092,971 Shares, 590,137 Shares, 293,628 Shares, respectively.</footnote>
        <footnote id="F7">High River directly wrote 1,208,442 European-style put options (the &quot;Put Options&quot;), Icahn Partners directly wrote 1,857,034 Put Options, Icahn Master directly wrote 2,092,971 Put Options, Icahn Master II directly wrote 590,137 Put Options, and Icahn Master III directly wrote 293,628 Put Options.  On March 1, 2011, upon exercise of the Call Options, all of the Put Options terminated.</footnote>
        <footnote id="F8">The options first became exercisable starting on various dates ranging from November 10, 2010 to November 16, 2010.</footnote>
        <footnote id="F9">The exercise price upon exercise of the call option was $2.90.  In addition, the premium paid for the call option was $1.69.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>CARL C. ICAHN</signatureName>
        <signatureDate>2011-03-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>ICAHN ENTERPRISES HOLDINGS L.P.</signatureName>
        <signatureDate>2011-03-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>BECKTON CORP.</signatureName>
        <signatureDate>2011-03-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>ICAHN ENTERPRISES G.P. INC.</signatureName>
        <signatureDate>2011-03-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>ICAHN CAPITAL LP</signatureName>
        <signatureDate>2011-03-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>IPH GP LLC</signatureName>
        <signatureDate>2011-03-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
