The 30-year-old has signed a five-year contract at San Siro on the back of six consecutive top-flight titles with Juve, the last three of which stand as half of domestic doubles on the back of triumphs in the 2015, 2016 and 2017 Coppa Italia finals.
The deal is reported to be worth €40 million ($58.6 million).
Bonucci formed part of European club football's most formidable backline alongside Giorgio Chiellini and Andrea Barzagli in Turin, in front of evergreen goalkeeping great Gianluigi Buffon.
Buffon, 39, is poised to retire after the 2018 FIFA World Cup, but Bonucci's shock sale hastens the break-up of Juventus's famed defensive unit.
Manchester City and Chelsea were reported to hold an interest in the former Inter Milan and Bari man 12 months ago, although he briefly quelled speculation over his future by penning a contract extension until 2021 at Juventus Stadium last December.
However, an argument with head coach Massimiliano Allegri during February's 4-1 win over Palermo resulted in Bonucci being dropped for the subsequent trip to Porto, while the player denied allegations of a half-time bust-up with Paulo Dybala and Barzagli during the UEFA Champions League final loss to Real Madrid in Cardiff – a rumour that has continued to fester and is unlikely to lose any of its potency now Milan have made their move.
Under the ambitious ownership of Yonghong Li's Rossoneri Sport Investment Lux group, Milan have embarked upon an ambitious programme of close-season spending, within which Bonucci is now the jewel.
He joins fellow new recruits Andre Silva, Ricardo Rodriguez, Hakan Calhanoglu, Andrea Conti and Mateo Musacchio to try and make a dent in Juventus's dominance of Italian football, a hegemony he was integral to establishing.