The 23-year-old, who joined Tottenham from Schalke 04 in January 2013, failed to hold down a regular starting spot with Spurs and was loaned out to Fulham in the January transfer window.
Holtby, a former Germany international, has also played for German clubs VfL Bochum and Mainz 05, scoring 16 goals in 99 Bundesliga games.
"Lewis is a highly skilled player with a great passing game who will do our game a lot of good," said Hamburg sports director Dietmar Beiersdorfer.
"With his 23 years he has a lot of experience already having played in the Bundesliga, the Premier League and the national team."
Hamburg, who needed a relegation playoff to stay up last season, lost 3-0 to promoted Paderborn on Saturday to remain on one point from two games this season.
(Reporting by Karolos Grohmann; Editing by Rex Gowar)
Share
