The announcement comes a day after the Saints signed kicker Garrett Hartley to a five-year contract.
Thomas, part of the team which won Super Bowl following the 2009 season, joined the Saints as an undrafted free agent in 2007, coming out of the University of Illinois.
The running back has led the club in rushing over that four-year period, gaining 1,939 yards with 18 rushing touchdowns in 47 regular season games, 15 of them starts.
An ankle injury limited Thomas to just six games in 2010 but his return bolsters a strong running game for the Saints, who also have Reggie Bush and Chris Ivory in the backfield.
Financial terms of the deal were not released.
(Reporting by Simon Evans in Miami; Editing by Steve Ginsburg)