The 30-year-old, who can play as a deep-lying striker or in midfield, returns to Craven Cottage having previously joined the west London club in January 2007 from New England Revolution.
Dempsey, who moved to Seattle from Tottenham Hotspur in the close season, scored 60 goals in 225 games in over five seasons at Fulham which included reaching the 2010 Europa League Final.
He was voted Fulham's player of the season in 2011-12 for the second straight season before moving to Spurs and the club will be hoping he can quickly recapture his best form as they battle against relegation under new manager Rene Meulensteen.
Fulham are second from bottom of the table with 13 points from 17 games, level on points with Crystal Palace and one point from the safety zone with Sunderland three points adrift. They visit Norwich City on Thursday and Hull City on Saturday.
(Reporting by Ken Ferris; Editing by Mark Lamport-Stokes)
