Milan confirmed the transfer, which came as a surprise, on their official website.
The German-born 26-year-old Ghana international, who has also played for Hertha Berlin, Portsmouth and Tottenham Hotspur and is the half brother of Bayern Munich's Jerome Boateng, joined Milan in 2010.
Schalke beat Greece's PAOK 4-3 on aggregate this week to book their spot in the Champions League group stage this season where they will face Chelsea, Basel and Steaua Bucharest.
- Sports & Recreation
- Jerome Boateng
- Hertha Berlin
- Tottenham Hotspur
- Bayern Munich