The 23-year-old, who is Chris Hughton's sixth summer signing, scored 21 goals in 71 appearances for FC Twente after starting his career with Feyenoord.
Fer, who has two full caps, captained Holland at Under-17 and Under-19 levels and he was a key part of their European Under-21 Championship team which made it to the semi-finals earlier this summer, scoring twice during the group stages.
He joins Javier Garrido, Ricky Van Wolfswinkel, Nathan Redmond, Martin Olsson and Carlo Nash in boosting Hughton's options for the 2013-14 Barclays Premier League season.
Hughton said: "We are really pleased to have acquired the signature of a player with the qualities that Leroy has.
"He's a very versatile Dutch international and at 23 he has that pleasing balance of having acquired experience playing over 100 games in the Dutch League - but still having plenty of time ahead of him to continue his development as a player here with us at Norwich City."