The 27-year-old joined the Black Cats in 2013 on an initial loan deal from Liverpool, and made his move there permanent in 2015.
It hasn't been a particularly good stint for the former Chelsea man at the Stadium of Light, after suffering relegation into the Championship in 2017.
A year on, Sunderland have gone down again and now find themselves languishing in League One, but Borini spent 2017/18 playing on loan at AC Milan. That's a move he will now make permanent, after impressing enough at San Siro in his 39 appearances in all competitions.
Borini proved to be a very useful signing for the Rossoneri, with Vincenzo Montella and Gennaro Gattuso utilising him in a number of different positions.
There have been no reports about whether the transfer involves a fee. If there is one though, it's likely not to be an awful lot.
Milan are expected to chop and change their squad again this summer, after failing to qualify for the Champions League, but Borini clearly managed to do enough to prompt the club into buying him ahead of the 2018/19 season.
Another player to leave the Stadium of Light permanently this summer will be winger Jeremain Lens, who spent last season on loan Besiktas.