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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2019-09-10</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001157806</issuerCik>
        <issuerName>ENCANA CORP</issuerName>
        <issuerTradingSymbol>ECA</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001788621</rptOwnerCik>
            <rptOwnerName>McCracken Brendan Michael</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>500 CENTRE STREET SE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CALGARY</rptOwnerCity>
            <rptOwnerState>A0</rptOwnerState>
            <rptOwnerZipCode>T2P5S2</rptOwnerZipCode>
            <rptOwnerStateDescription>ALBERTA, CANADA</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP, Corp Dev &amp; Ext Relations</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Shares</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12400</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Shares</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>801</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>RRSP</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Options (Tandem Stock Appreciation)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>11.20</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2022-03-13</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>42081</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Options (Tandem Stock Appreciation)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.15</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2023-03-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>101471</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Options (Tandem Stock Appreciation)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>11.72</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2024-02-27</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>28495</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Options (Tandem Stock Appreciation)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>10.85</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2025-02-26</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>22755</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Options (Tandem Stock Appreciation)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>7.16</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2026-03-08</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>54246</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Options (Tandem Stock Appreciation)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.54</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <value>2026-09-10</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>56092</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Rights (Restricted Share Unit)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>11.72</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2020-02-27</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F7"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>14595</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Rights (Restricted Share Unit)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>10.85</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2021-02-26</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F7"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>12076</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Rights (Restricted Share Unit)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>7.16</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2022-03-08</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F7"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>25114</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">30% vested on March 13, 2016, 30% vested on March 13, 2017 and 40% vested on March 13, 2018, subject to the grantee's active employment on such dates.</footnote>
        <footnote id="F2">30% vested on March 3, 2017, 30% vested on March 3, 2018 and 40% vested on March 3, 2019, subject to the grantee's active employment on such dates.</footnote>
        <footnote id="F3">30% vested on February 27, 2018, 30% vested on February 27, 2019 and 40% vest on February 27, 2020, subject to the grantee's active employment on such dates.</footnote>
        <footnote id="F4">30% vested on February 26, 2019, 30% vest on February 26, 2020 and 40% vest on February 26, 2021, subject to the grantee's active employment on such dates.</footnote>
        <footnote id="F5">30% vest on March 8, 2020, 30% vest on March 8, 2021 and 40% vest on March 8, 2022, subject to the grantee's active employment on such dates and the terms and conditions of the Omnibus Incentive Plan of Encana Corporation and applicable grant agreement.</footnote>
        <footnote id="F6">30% vest on September 10, 2020, 30% vest on September 10, 2021 and 40% vest on September 10, 2022, subject to the grantee's active employment on such dates and the terms and conditions of the Omnibus Incentive Plan of Encana Corporation and applicable grant agreement.</footnote>
        <footnote id="F7">Each Restricted Share Unit (each, a &quot;RSU&quot;) is the economic equivalent of one common share of Encana Corporation. RSUs are subject to the officer's active employment on the vesting date and the terms and conditions of the Omnibus Incentive Plan of Encana Corporation and applicable grant agreement.</footnote>
        <footnote id="F8">The grants were issued in Canadian dollars. The price has been converted to U.S. dollars using the Bank of Canada exchange rate on the grant date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/Dawna Gibb by Power of Attorney</signatureName>
        <signatureDate>2019-09-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
