The 33-year-old France international has signed a two-year contract with the Italian champions after ending his 8½-half-year stint at Old Trafford.
Evra had been linked with a move to Juve earlier this summer but ended up signing a one-year contract extension with United in May, with his previous deal poised to run out. However, the experienced full-back, who would have been competing for a starting berth against summer signing Luke Shaw, has now decided to move on anyway after nine hugely successful seasons at United.
Evra said: "It is the biggest decision of my career as this club is, and will always remain, in my heart. Joining and playing for Manchester United has been a privilege, a dream come true and my time here has been so memorable. Every single time I stepped out in that shirt I knew I was playing for the badge and for a very special club."