The 31-year-old, capped 80 times by Italy, spent seven years at Juventus before joining Milan for one season.
The move is set to be part of a deal that will take Argentina forward Gonzalo Higuain in the opposite direction from Juve to Milan.
Higuain will also undergo a medical on Thursday, Milan said on Twitter.
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Bonucci's return to Juve would reunite him with Andrea Barzagli and Giorgio Chiellini, who together formed the so-called 'BBC' defence, which proved to be one of Europe's most formidable rearguards.
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