The striker becomes the club's fifth signing of the close-season, with the Sunderland forward swapping the Championship for Serie A
Fabio Borini has completed his move to AC Milan from Sunderland on loan with an obligation to purchase.
The forward's switch to San Siro was confirmed Friday as he signed the contract in a Facebook live video.
Borini, who will reportedly cost €6million when Milan make the move permanent, is the club's fifth signing ahead of the 2017-18 season, following the arrivals of Andre Silva, Ricardo Rodriguez, Mateo Musacchio and Franck Kessie.
The 26-year-old's switch to the Serie A heavyweights comes as a surprise after he only scored twice in 24 Premier League appearances as Sunderland finished bottom last season and suffered relegation to the Championship.