Schalke sign Boateng from AC Milan
The Italian side confirmed the deal on its website Friday, when Schalke presented the player at a news conference.
''I am very happy to be home in Germany, in the best league in the world,'' Boateng said.
Schalke general manager Horst Heldt said the club had signed the 26-year-old Boateng to a four-year deal.
''He is a player of extraordinary quality and a leading personality,'' Heldt said.
Heldt said it was easy to convince Boateng to switch to the Gelsenkirchen-based club - the difficulty was in convincing Milan to sell him.
Boateng scored twice as Milan qualified for the Champions League with a 3-0 win over PSV Eindhoven on Wednesday. Under UEFA rules, he remains eligible to play for Schalke in the competition proper.
''The deal for Boateng got put together lightning quick,'' Milan vice president Adriano Galliani said. ''It started yesterday morning and it satisfies all sides.''
While he was a key player in Milan's midfield, Boateng had a rocky time in Italy. In January, he was subjected to racist chants during a friendly with lower-division club Pro Patria, and he responded by walking off the field followed by the rest of his teammates.
Boateng scored 10 goals in 74 league games for Milan since joining in 2010. He previously played for English clubs Tottenham and Portsmouth, as well as Borussia Dortmund and Hertha Berlin.
He represented Germany at youth level but opted to play for the country of father ahead of the 2010 World Cup.
''He has a lot of international experience and will greatly help our team,'' Schalke coach Jens Keller said.