The 35-year-old, who made his debut for the club in 2007, had earned a short-term deal after impressing Erik ten Hag when training with them after being released by Leicester.
United have now acted to keep the veteran at Old Trafford for the season, with the Northern Ireland international becoming their third signing of deadline day after Altay Bayindir and Sergio Reguilon.
With Raphael Varane and Luke Shaw injured, Ten Hag is without two of his centre-back options, and Evans has been registered in time to feature at Arsenal on Sunday. That would mark his first appearance for United since 2015.
Evans said: “I am delighted to join Manchester United, a place which has always felt like home. This club and the fans have shaped me as a player and as a person since I was 15 years old, and I am delighted to be back where it all began as part of this talented squad.
“I’ve really enjoyed working under the manager and his coaching staff since the start of pre-season, and I’m looking forward to helping this group achieve success together at this fantastic football club.”
Evans scored seven goals in 198 appearances in his first spell at Old Trafford, winning three Premier League titles and one Champions League trophy, before spending three seasons at West Bromwich Albion and five at Leicester.
United’s director of football John Murtough said: “Jonny is a winner who has experienced everything in the game. These characteristics are invaluable to the culture that we’re building here.
“His attitude and work ethic are second to none, and Erik and the coaching staff know he’s someone they can rely on whenever needed this season. We all welcome Jonny back and hope he can add further achievements to the success he has already enjoyed with the club.”