The former Schalke man will spend the remainder of the current campaign at Craven Cottage, the clubs announced on Friday.
"Fulham Football Club can this afternoon confirm the loan signing of Lewis Holtby from Tottenham Hotspur, for the remainder of the season," read a statement on Fulham's official website.
Holtby scored against Fulham for Spurs at Craven Cottage during a 2-1 win for the visitors - in what was Rene Meulensteen's first game in charge.
The Germany international has struggled to nail down a regular starting spot at White Hart Lane since moving from the Veltins Arena in January 2013.
He has made only six Premier League starts this season, scoring three goals in all competitions since his arrival.
Holtby joins a Fulham side in 19th place in the Premier League, a point off the bottom of the table.
