Pirlo joins Juventus on three-year deal
Pirlo, 32, struggled with injuries and form for Milan last season and will become a rare example of a player who has turned out for both of Italy's two most successful teams.
Juve said in a statement he had signed until June 30, 2104 having undergone a medical.
The Italy midfielder's arrival at Juve could signal a complication in the Turin side's bid to tie up a permanant deal for on-loan Liverpool midfielder Alberto Aquilani, who plays in a very similar position.
Juve could only finish seventh for the second straight year this term after a series of transfer flops as they battle to regain their former glory following a 2006 match-fixing demotion.
They are still without a coach with media saying Siena boss and former Juve midfielder Antonio Conte will take over from the sacked Luigi Delneri.
Pirlo's struggles this term meant he was no longer a first-team regular under Milan coach Massimiliano Allegri with his lack of mobility and a habit of staying around the centre circle pinging long passes failing to enthuse the boss.