The midfielder has been out of favour at White Hart Lane and was training at Fulham on Friday morning ahead of a proposed move.
Holtby's agent later confirmed to Press Association Sport that his client had made the switch on loan until the summer.
"Lewis will stay in London and has joined Fulham on loan until the end of the season," Holtby's agent Marcus Noack said.
The move did not go through without acrimony, though.
Noack accused Tottenham of pricing Holtby out of a permanent transfer back to his native Germany.
"Fulham has put much effort into Lewis," he said.
"It is a very well-run club with an excellent manager Rene Meulensteen. The system and philosophy how Fulham plays fits well with Lewis.
"He will have match practice in the second half of the season in his favourite position and then we'll see how things are going by summer.
"A return to the Bundesliga has been shattered due to circumstances we couldn't control."
Holtby joined Tottenham from Schalke 12 months ago for just £1.25million, but the midfielder's camp claimed Spurs wanted £8-10m for the player.