The 31-year-old joins The Black and Greens from German club Eintracht Frankfurt, who acquired his services last summer.
"The Sassuolo Calcio Sports Union announces that it has completed the following market operation: Boateng Kevin-Prince ( midfielder) acquired permanently from Eintracht Frankfurt," the Italian side have announced.
"The player will be presented to the press tomorrow Friday, July 6 at 3 pm at the Seminar Room of the Mapei office in Via Valle d'Aosta, 46 in Sassuolo."
In his only season with Frankfurt, Boateng made 31 Bundesliga appearances, involving 27 starts, and scored six goals to help the club finish eighth on the table.
He also had five outings to his name as The Eagles clinched the German Cup.
"With the cup win, the circle in Germany has ended for me, and I leave Eintracht with a positive feeling," Boateng said, as reported by Frankfurt's official website.
"It was an outstanding year for me and I will never forget the time in Frankfurt.
"My thanks to my teammates, the coaching team, the team in the background and the sporting leadership, especially [board representative] Fredi Bobic, and of course the fans for their great support."
Boateng first played in Italy when he signed for Milan in 2010 after an impressive World Cup campaign in South Africa.
He left in 2013 with one Serie A title and an Italian Super Cup gold, before returning for a short stint in 2016.