Holtby has never established himself as a regular starter at White Hart Lane since moving from Schalke in January 2013.
The 23-year-old spent the second half of last season on loan at Fulham, where he was unable to stop the west London side being relegated from the Premier League.
And Holtby has now made the move back to the Bundesliga with Hamburg for the duration of the 2014-15 campaign.
The German club have also offloaded two players, with midfielders Per Skjelbred and Milan Badelj joining Hertha BSC and Fiorentina respectively.
Skjelbred spent last season on loan in the German capital and has put pen to paper on a three-year contract.
Badelj, meanwhile, has agreed a four-year contract with Fiorentina.