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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2015-04-29</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001113256</issuerCik>
        <issuerName>MERITOR INC</issuerName>
        <issuerTradingSymbol>MTOR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001641729</rptOwnerCik>
            <rptOwnerName>Speed Robert H.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MERITOR, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2135 WEST MAPLE ROAD</rptOwnerStreet2>
            <rptOwnerCity>TROY</rptOwnerCity>
            <rptOwnerState>MI</rptOwnerState>
            <rptOwnerZipCode>48084</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Pres., Aftmkt &amp; Trailer, CPO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>41784</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Includes 38,413 Restricted Share Units (&quot;RSUs&quot;), each of which represents the right to receive one share of common stock of Meritor, Inc. (&quot;Meritor&quot;) upon the vesting date, which occurs three years from the date of grant or earlier, upon termination of employment with Meritor under certain circumstances.</footnote>
    </footnotes>

    <remarks>Exhibit List:  Exhibit 24-Power of Attorney</remarks>

    <ownerSignature>
        <signatureName>/s/ Robert H. Speed, By: Sandra J. Quick, Attorney-in-Fact</signatureName>
        <signatureDate>2015-05-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
