Capped 57 times by Germany, scoring eight goals, the 29-year-old has signed a five-year deal.
An official club statement this morning reads:
The versatile wing-back/midfielder was born in Berlin and started his career at Bayern Munich in 1990. He transferred to AC Milan in 1997 before moving to Middlesbrough for £4million in August 1999. Liverpool paid £5.5million for him in August last year.
Christian Ziege has completed his transfer to the club from Liverpool.