Coming through the youth system, Boateng began his career at Hertha BSC, before joining Tottenham Hotspur in England. After a short loan spell with Borussia Dortmund, Boateng moved to Portsmouth. In 2010 he joined Serie A side Milan, winning both the league and Supercoppa Italiana titles in his first season. Boateng left Milan after three seasons, returning to Germany joining Schalke 04. In January 2016 he returned to Milan, remaining for six months, before moving to Spanish club Las Palmas. Boateng made his third return to Germany in 2017, playing for Eintracht Frankfurt, before joining Italian side Sassuolo. After a short loan to Barcelona, with whom he won a La Liga title, he moved back to Italy, joining Fiorentina. In January 2020 he was sent on a six-month loan to Turkish side Beşiktaş, and in September 2020 he moved to Monza.