The 29-year-old striker, who came through the youth ranks at the San Siro, made his professional debut for the Rossoneri against Piacenza in 2003.
He was then loaned out to Prato, Lumezzane and Rimini before securing a permanent transfer to Cagliari in 2007.
After an initial loan spell at Juventus, the Italian international managed to persuade the Bianconeri to outlay €15.5m (£13.2m) for his services two years ago.
Matri, who has been capped five times by Italy, had been linked with Everton and Napoli over the summer, but the Milan Channel announced his return to Diavolo yesterday evening.
Since arriving in Turin, he has scored 29 goals in all competitions.