The 22-year-old right back, who signed a pre-contract agreement with the Dons earlier this month, has moved now after Aberdeen and Livingston agreed a fee.
Dons boss Steve Paterson told the official AFC website "I've been an admirer of Michael's ever since his early days at Aberdeen. When I knew he was first available nearly three years ago, I tried to take him to Inverness but he chose to go to Livingston. I've had to wait a long time to finally get my man, but I'm convinced it will be well worth the wait as he's a versatile player who is comfortable in a number of positions. I'm happy we now have a nucleus of talented Scottish defenders at the club who can take us forward."
Hart said of his return "Having grown up with the majority of the players who are now first team regulars, settling in will be very easy. I think it's an exciting time to be coming back to Pittodrie as Steve Paterson and Duncan Shearer have brought new life to the club. I like the way they think about the game so the outlook for the Dons is a positive one."