The 29-year-old arrives for a reported fee of 9.8 million euros to become Nigel Pearson's third summer signing after impressing in the Bundesliga for the past four years.
Leicester City has agreed a deal to sign Shinji Okazaki from Mainz.
The Japan international has signed a four-year contract with at the KC Stadium, while his move is subject to a work permit and international clearance.
Okazaki, a striker, becomes Nigel Pearson's third summer signing following the arrival of defender
Christian Fuchs from Schalke. Robert Huth has also turned his loan from Stoke City into a permanent deal.