Holtby, 23, who has a German mother and English father and has played three times for Germany, joined Tottenham from Schalke 04 in the January transfer window a year ago but has been unable to establish a regular place in the first-team.
He scored his only Premier League goal with a spectacular long-range strike at Craven Cottage to give Spurs a 2-1 win over Fulham last month. He also scored twice for Spurs in the Europa League.
A Fulham spokesman said that he had trained with his new club on Friday and would be staying until the end of the season.
Fulham also confirmed that their Northern Ireland international defender Aaron Hughes has joined Championship club Queens Park Rangers on loan, but could not confirm reports that Dimitar Berbatov, the Bulgarian international striker, had been given permission to speak to Monaco.
Berbatov has also been linked with a move back to Spurs, where he played between 2006 and 2008 although a move to Monaco looks more likely.
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- Schalke 04