The former Manchester United trainee 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.
However, his first-team appearances were restricted under former boss 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.
Source: PA
Source: PA