His agent Markus Noack said that Holtby would consider his long-term future in the summer, but praised the "effort" that Fulham have put into signing him for the rest of the season.
As yet there is no indication whether the deal includes a clause for Fulham to make it permanent, but they are believed to have paid £1m to secure the player.
"Fulham have put much effort into Lewis," BBC Sport quotes Noack as saying. "It is a very well-run club with an excellent [head coach] in Rene Meulensteen. The system and philosophy, how Fulham play 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 Spurs last January from Schalke 04 and will take the number 10 shirt at Fulham.