The Juventus striker deemed surplus to requirements at the Bianconeri has rejoined the Rossoneri in a deal believed to be around €11 million (£9.4m)
Matri, 29, is believed to have signed on a four-year deal at Milan, the club he played for as a youngster before having spells at Cagliari and Juventus, in a deal reportedly worth €11 million (£9.4m).
The striker has fallen down the pecking order at the Bianconeri, with other attackers Fernando Llorente and Carlos Tevez joining Antonio Conte's men earlier this summer.
But, with the Rossoneri securing their place in the Champions League proper, having seen off the challenge of PSV and Giampaolo Pazzini's return still to be confirmed, coach Massimiliano Allegri has bolstered his squad with the capture of the forward.
Matri made 83 appearances for Juventus, scoring 29 goals.