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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-07-17</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000808450</issuerCik>
        <issuerName>NAVISTAR INTERNATIONAL CORP</issuerName>
        <issuerTradingSymbol>NAV</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>0000928464</rptOwnerCik>
            <rptOwnerName>HIGH RIVER LIMITED PARTNERSHIP</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>0001313666</rptOwnerCik>
            <rptOwnerName>ICAHN PARTNERS 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>0001322827</rptOwnerCik>
            <rptOwnerName>ICAHN PARTNERS MASTER FUND LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O WALKER HOUSE</rptOwnerStreet1>
            <rptOwnerStreet2>87 MARY STREET</rptOwnerStreet2>
            <rptOwnerCity>GEORGE TOWN, GRAND CAYMAN</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>KY1-9001</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001414181</rptOwnerCik>
            <rptOwnerName>ICAHN PARTNERS MASTER FUND II L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O WALKER HOUSE</rptOwnerStreet1>
            <rptOwnerStreet2>87 MARY STREET</rptOwnerStreet2>
            <rptOwnerCity>GRAND CAYMAN, CAYMAN ISLANDS</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>KY1-9001</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001414179</rptOwnerCik>
            <rptOwnerName>ICAHN PARTNERS MASTER FUND III L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O WALKER HOUSE</rptOwnerStreet1>
            <rptOwnerStreet2>87 MARY STREET</rptOwnerStreet2>
            <rptOwnerCity>GRAND CAYMAN, CAYMAN ISLANDS</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>KY1-9001</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</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</value>
            </securityTitle>
            <transactionDate>
                <value>2012-07-17</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>10640</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>23.01</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9276228</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"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">High River Limited Partnership (&quot;High River&quot;) directly beneficially owns 1,855,245 Shares, Icahn Partners LP (&quot;Icahn Partners&quot;) directly beneficially owns 2,741,030 Shares, Icahn Partners Master Fund LP (&quot;Icahn Master&quot;) directly beneficially owns 2,993,986 Shares, Icahn Partners Master Fund II LP (&quot;Icahn Master II&quot;) directly beneficially owns 1,169,834 Shares, and Icahn Partners Master Fund III LP (&quot;Icahn Master III&quot;) directly beneficially owns 516,133 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>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>CARL C. ICAHN</signatureName>
        <signatureDate>2012-07-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
