The 23-year-old winger's new deal keeps him at the Championship club until June 2016.
He made 29 appearances for the Whites in all competitions last season.
Kacaniklic previously had loan spells at Watford and Burnley, after joining the club as part of a deal which took Paul Konchesky to Liverpool in 2010.
Sweden international Alexander Kacaniklic has joined FC Copenhagen on a season-long loan after signing a contract extension at Fulham.