The defender, who returns to Germany after an 11-year absence and started his career as a 16-year-old with Arsenal, has joined the club after passing a medical on Monday and signing a two-year contract.
He was a free agent after being released by Fulham a year ago. "I am certain that this club suits me and I suit this club," said the 27-year-old on the club's website.
"I am looking forward to the tasks ahead, the new league and the challenge we face as a team."