The 30-year-old, previously linked with Manchester City and Chelsea, has been a key part of the squad which has won the last six titles but looks ready to opt for a new challenge with the Rossoneri.
A day earlier the Lazian centre-back - a 70-time Italy international - sparked intrigue among Juve fans by tweeting: "Back to work. I'm ready to start again!"
Their initially faint fears appear to be well-founded as the player for so long part of a solid three-man defence alongside Giorgio Chiellini and Andrea Barzagli is about to depart the Allianz Stadium.
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In a video published on Twitter, Bonucci was seen arriving at the Casa Milan headquarters at around 2pm local time (1pm BST) on Friday, with his car greeted by around 200 excited fans as it drove past.
It marks a return to San Siro for Bonucci, who began his career with Milan's foes Inter, but he made only one substitute appearance in the league along with a handful of Coppa Italia matches.
He had loan spells at Serie B sides Treviso and Pisa before he joined Genoa in 2009. However, he was almost immediately transferred to Bari in a co-ownership deal.
He was ever-present in his one season with Bari, forming a strong central-defensive partnership with Andrea Ranocchia, before being snapped up by Juve.
It is the latest move in an ambitious summer of recruitment by Milan's new Chinese owners, who have already funded the purchases of Andre Silva, Ricardo Rodriguez, Hakan Calhanoglu, Andrea Conti, Mateo Musacchio, Franck Kessie and Fabio Borini.
Lazio midfielder Lucas Biglia is expected to follow and the club have even been linked with Borussia Dortmund's prolific striker Pierre-Emerick Aubameyang.
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