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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-05-15</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001274848</rptOwnerCik>
            <rptOwnerName>FILIPOV STEVE</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TEREX CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>200 NYALA FARM ROAD</rptOwnerStreet2>
            <rptOwnerCity>WESTPORT</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06880</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President, Devl Mkts Strt Act</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01</value>
            </securityTitle>
            <transactionDate>
                <value>2008-05-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>9193</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>72.36</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>77165</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01</value>
            </securityTitle>
            <transactionDate>
                <value>2008-05-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>I</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>0</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3740</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>401(k) plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents average sale price.  Shares sold at various prices ranging from $71.80 to $72.90 as follows (shares at dollar price per share): 100 @ $71.80; 100 @ $71.85; 200 @ $72.00; 100 @ $72.01; 200 @ $72.02; 100 @ $72.03; 300 @ $72.04; 300 @ $72.05; 100 @ $72.06; 100 @ $72.08; 500 @ $72.10; 100 @ $72.11; 200 @ $72.12; 200 @ $72.13; 400 @ $72.16; 100 @ $72.17; 200 @ $72.19; 200 @ $72.20; 400 @ $72.21; 100 @ $72.23; 100 @ $72.26; 200 @ $72.28; 200 @ $72.30; 100 @ $72.31; 100 @ $72.34; 100 @ $72.35; 100 @ $72.40; 200 @ $72.41; 400 @ $72.42; 100 @ $72.43; 200 @ $72.44; 100 @ $72.45; 100 @ $72.47; 100 @ $72.48; 100 @ $72.49; 100 @ $72.51; 100 @ $72.54; 200 @ $72.55; 200 @ $72.59; 300 @ $72.60; 100 @ $72.62; 100 @ $72.63; 100 @ $72.64; 100 @ $72.65; 100 @ $72.67; 100 @ $72.70; 100 @ $72.72; 193 @ $72.74; 100 @ $72.76; 100 @ $72.77; 300 @ $72.78; 100 @ $72.80; 100 @ $72.85; 100 @ $72.86; 300 @ $72.88 and 100 @ $72.90</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Scott J. Posner, power of attorney</signatureName>
        <signatureDate>2008-05-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2008-05-15
0000097216
TEREX CORP
TEX
0001274848
FILIPOV STEVE
TEREX CORPORATION
200 NYALA FARM ROAD
WESTPORT
CT
06880
0
1
0
0
President, Devl Mkts Strt Act
Common Stock, par value $.01
2008-05-15
4
S
0
9193
72.36
D
77165
D
Common Stock, par value $.01
2008-05-15
5
I
0
0
0
A
3740
I
401(k) plan
Represents average sale price.  Shares sold at various prices ranging from $71.80 to $72.90 as follows (shares at dollar price per share): 100 @ $71.80; 100 @ $71.85; 200 @ $72.00; 100 @ $72.01; 200 @ $72.02; 100 @ $72.03; 300 @ $72.04; 300 @ $72.05; 100 @ $72.06; 100 @ $72.08; 500 @ $72.10; 100 @ $72.11; 200 @ $72.12; 200 @ $72.13; 400 @ $72.16; 100 @ $72.17; 200 @ $72.19; 200 @ $72.20; 400 @ $72.21; 100 @ $72.23; 100 @ $72.26; 200 @ $72.28; 200 @ $72.30; 100 @ $72.31; 100 @ $72.34; 100 @ $72.35; 100 @ $72.40; 200 @ $72.41; 400 @ $72.42; 100 @ $72.43; 200 @ $72.44; 100 @ $72.45; 100 @ $72.47; 100 @ $72.48; 100 @ $72.49; 100 @ $72.51; 100 @ $72.54; 200 @ $72.55; 200 @ $72.59; 300 @ $72.60; 100 @ $72.62; 100 @ $72.63; 100 @ $72.64; 100 @ $72.65; 100 @ $72.67; 100 @ $72.70; 100 @ $72.72; 193 @ $72.74; 100 @ $72.76; 100 @ $72.77; 300 @ $72.78; 100 @ $72.80; 100 @ $72.85; 100 @ $72.86; 300 @ $72.88 and 100 @ $72.90
/s/ Scott J. Posner, power of attorney
2008-05-19