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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-06-30</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000813762</issuerCik>
        <issuerName>ICAHN ENTERPRISES L.P.</issuerName>
        <issuerTradingSymbol>IEP</issuerTradingSymbol>
    </issuer>

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

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Depositary Units</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </securityTitle>
            <transactionDate>
                <value>2015-06-30</value>
                <footnoteId id="F3"/>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F5"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1791781</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>86.96</value>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>112285454</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Please see footnotes</value>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                    <footnoteId id="F8"/>
                    <footnoteId id="F9"/>
                    <footnoteId id="F10"/>
                    <footnoteId id="F11"/>
                    <footnoteId id="F12"/>
                    <footnoteId id="F13"/>
                    <footnoteId id="F14"/>
                    <footnoteId id="F15"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Depository Units representing limited partner interests in Icahn Enterprises L.P. (the &quot;Issuer&quot;).</footnote>
        <footnote id="F2">Comprised of Depositary Units held indirectly through Barberry Corp., CCI Offshore LLC, CCI Onshore LLC, Gascon Partners, High Coast Limited Partnership, Highcrest Investors LLC, Tramore LLC, Modal LLC and Thornwood Associates Limited Partnership.</footnote>
        <footnote id="F3">Represents the date on which dividend amounts are determined based on the election of each holder and the volume weighted average trading price of units on NASDAQ during ten consecutive trading days following the election date.</footnote>
        <footnote id="F4">Represents the amount foregone in exchange for each Depository Unit received as a dividend.</footnote>
        <footnote id="F5">The reporting person received 1,791,781 Depositary Units as a payment-in-kind dividend on 110,493,673 Depositary Units owned on the dividend record date in a transaction exempt from Section 16(b) liability pursuant to Rule 16(b)-3(e) promulgated under the Securities Exchange Act of 1934, as amended.</footnote>
        <footnote id="F6">Barberry Corp. (&quot;Barberry&quot;) beneficially owns 3,710,275 Depository Units. Carl C. Icahn beneficially owns 100% of Barberry. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, Mr. Icahn (by virtue of his relationship to Barberry) may be deemed to indirectly beneficially own the Depository Units which Barberry owns. Mr. Icahn disclaims beneficial ownership of such Depository Units except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F7">CCI Offshore LLC (&quot;CCI Offshore&quot;) beneficially owns 2,385,486 Depository Units. Barberry is the sole member of CCI Offshore. Carl C. Icahn beneficially owns 100% of Barberry. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, each of Mr. Icahn and Barberry (by virtue of their relationships to CCI Offshore) may be deemed to indirectly beneficially own the Depository Units which CCI Offshore owns. Each of Mr. Icahn and Barberry disclaims beneficial ownership of such Depository Units except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F8">CCI Onshore LLC (&quot;CCI Onshore&quot;) beneficially owns 18,895,641 Depository Units. High Coast Limited Partnership (&quot;High Coast&quot;) is the sole member of CCI Onshore. Little Meadow Corp. (&quot;Little Meadow&quot;) is the general partner of High Coast. Carl C. Icahn beneficially owns 100% of Little Meadow. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, each of Mr. Icahn, Little Meadow and High Coast (by virtue of their relationships to CCI Onshore) may be deemed to indirectly beneficially own the Depository Units which CCI Onshore owns. Each of Mr. Icahn, Little Meadow and High Coast disclaims beneficial ownership of such Depository Units except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F9">Gascon Partners (&quot;Gascon&quot;) beneficially owns 16,621,729 Depository Units. Little Meadow is the managing general partner of Gascon. Carl C. Icahn beneficially owns 100% of Little Meadow. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, each of Mr. Icahn and Little Meadow (by virtue of their relationships to Gascon) may be deemed to indirectly beneficially own the Depository Units which Gascon owns. Each of Mr. Icahn and Little Meadow disclaims beneficial ownership of such Depository Units except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F10">High Coast Limited Partnership (&quot;High Coast&quot;) beneficially owns 43,334,660 Depository Units. Little Meadow is the general partner of High Coast. Carl C. Icahn beneficially owns 100% of Little Meadow. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, each of Mr. Icahn and Little Meadow (by virtue of their relationships to High Coast) may be deemed to indirectly beneficially own the Depository Units which High Coast owns. Each of Mr. Icahn and Little Meadow disclaims beneficial ownership of such Depository Units except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F11">Highcrest Investors LLC (&quot;Highcrest&quot;) beneficially owns 12,910,248 Depository Units. Starfire Holding Corporation (&quot;Starfire&quot;) beneficially owns 99.5% of Highcrest. Carl C. Icahn beneficially owns 100% of Starfire. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, each of Mr. Icahn and Starfire (by virtue of their relationships to Highcrest) may be deemed to indirectly beneficially own the Depository Units which Highcrest owns. Each of Mr. Icahn and Starfire disclaims beneficial ownership of such Depository Units except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F12">Tramore LLC (&quot;Tramore&quot;) beneficially owns 8,450,837 Depository Units. Carl C. Icahn beneficially owns 100% of Tramore. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, Mr. Icahn (by virtue of his relationship to Tramore) may be deemed to indirectly beneficially own the Depository Units which Tramore owns. Mr. Icahn disclaims beneficial ownership of such Depository Units except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F13">Modal LLC (&quot;Modal&quot;) beneficially owns 744,992 Depository Units. Carl C. Icahn beneficially owns 100% of Modal. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, Mr. Icahn (by virtue of his relationship to Modal) may be deemed to indirectly beneficially own the Depository Units which Modal owns. Mr. Icahn disclaims beneficial ownership of such Depository Units except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F14">Thornwood Associates Limited Partnership (&quot;Thornwood&quot;) beneficially owns 5,231,586 Depository Units. Barberry is the general partner of Thornwood. Carl C. Icahn beneficially owns 100% of Barberry. Pursuant to Rule 16a-1(a)(2) under the Exchange Act, each of Mr. Icahn and Barberry (by virtue of their relationships to Thornwood) may be deemed to indirectly beneficially own the Depository Units which Thornwood owns. Each of Mr. Icahn and Barberry disclaims beneficial ownership of such Depository Units except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F15">Mr. Icahn may be deemed to indirectly beneficially own the Depository Units owned by Gail Golden, his wife. Mr. Icahn disclaims beneficial ownership of such Depositary Units for all purposes.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>CARL C. ICAHN</signatureName>
        <signatureDate>2015-07-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
