The former Manchester United player has signed a three-year contract at the City Ground where he is reunited with manager Steve McClaren, whom he played under at Middlesbrough.
Greening, 32, moved to Fulham from West Brom last summer after a successful season-long loan at the London club.
Happy days: Greening will be hoping to play more first team football with Nottingham Forest
However, his first-team appearances were restricted under Mark Hughes and he started only six Barclays Premier League games last season.
Greening becomes McClaren's second signing of the summer after Andy Reid returned to the club on a free transfer at the start of this month.
Meanwhile, former Tottenham and Hull defender Anthony Gardner is on the verge of joining Reading. The 29-year-old, who spent most of last season on loan at Crystal Palace, is available after his release from Hull.
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