Boateng, who will wear the number nine shirt, was unveiled as a Schalke player on Friday afternoon.
"I'm delighted to join a club like Schalke 04," Bundesliga.com quotes Boateng as saying.
"The Bundesliga's the best league in the world and the best players belong there. It was an easy decision for me. I'm happy to be home."
Boateng opted to play for Ghana and has appeared nine times for the African side.