The 31-year-old player who had been loaned out to Lazio, Juventus, Fiorentina and Genoa in recent seasons by Milan, signed a contract with Sassuolo for two years on Tuesday, but the fee was not disclosed, Xinhua news agency reported.
Matri's most successful periods were during Juventus where he won three league titles with the club. He also boasts seven caps with Italy.
Last year, Sassoulo ranked sixth in Serie A, one place above Milan, and Lazio where Matri played 31 matches on loan.