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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-05-07</periodOfReport>

    <issuer>
        <issuerCik>0000097216</issuerCik>
        <issuerName>TEREX CORP</issuerName>
        <issuerTradingSymbol>TEX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001193675</rptOwnerCik>
            <rptOwnerName>CHOLMONDELEY PAULA H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TEREX CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>200 NYALA FARM ROAD</rptOwnerStreet2>
            <rptOwnerCity>WESPORT</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06880</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01</value>
            </securityTitle>
            <transactionDate>
                <value>2009-05-07</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>175</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>15.38</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12816</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Purchase of shares by the reporting person in a retirement plan for the benefit of the reporting person.  The purchase transaction and sale transaction from 5/4/2009, previously reported on a Form 4, were matchable under Section 16(b) of the Securities Exchange Act of 1934. The reporting person had earlier agreed to pay to the Issuer the full amount of any profit realized in connection with such short-swing transactions, less transaction costs.  As the cost of shares purchased exceeded the proceeds from the shares sold, there were no short-swing profits.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Scott J. Posner, by Power of Attorney</signatureName>
        <signatureDate>2009-05-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0303
4
2009-05-07
0000097216
TEREX CORP
TEX
0001193675
CHOLMONDELEY PAULA H
C/O TEREX CORPORATION
200 NYALA FARM ROAD
WESPORT
CT
06880
1
0
0
0
Common Stock, par value $.01
2009-05-07
4
P
0
175
15.38
A
12816
D
Purchase of shares by the reporting person in a retirement plan for the benefit of the reporting person.  The purchase transaction and sale transaction from 5/4/2009, previously reported on a Form 4, were matchable under Section 16(b) of the Securities Exchange Act of 1934. The reporting person had earlier agreed to pay to the Issuer the full amount of any profit realized in connection with such short-swing transactions, less transaction costs.  As the cost of shares purchased exceeded the proceeds from the shares sold, there were no short-swing profits.
/s/ Scott J. Posner, by Power of Attorney
2009-05-08