The Algerian striker has signed a two-and-a-half-year contract at the City Ground.
Djebbour, 29, was set to join Forest last summer but the deal fell through. He has since been on loan with Turkish club Sivasspor, scoring twice in 10 league appearances this season.
Last term he netted 20 goals in 25 league games for his parent club Olympiacos.
Djebbour could make his debut in Forest's Championship match against Watford tomorrow night.