The move brings an end to Tielemans' three-year spell at Villa Park, with the 29-year-old having initially arrived in the Premier League with Leicester City in 2019. His departure comes shortly after helping Belgium reach the World Cup quarter-finals this summer.
Tielemans was a key figure in Aston Villa's run to Europa League glory last season, registering seven assists across the campaign and scoring the opening goal in the final against SC Freiburg. Statistically, he ranked among the Premier League's most progressive midfielders last term, breaking the most defensive lines per 100 passes of any attacker or midfielder in the division, while also making the second-most line-breaking passes under pressure.
Speaking after the move was completed, Tielemans said joining United was a defining moment in his career.
"It's hard to describe just how proud I am to join Manchester United. Signing for such a special club feels incredible, it is the culmination of years of dedication since I first fell in love with football.
"I have had the privilege of experiencing success in the game and it has only increased my determination to achieve more. The ambition from everyone at the club is extremely clear; we are all determined to push for the biggest trophies over the coming years."
United's director of football, Jason Wilcox, said the club were delighted to add a player of Tielemans' experience to the squad.
"Youri has consistently been one of the most outstanding midfielders in the Premier League throughout the past seven years. He has all of the technical qualities, as well as the ambition and mentality, to thrive at Manchester United. Youri's consistency is exceptional, and he will add further composure, creativity and leadership to our squad. We are delighted to welcome a player of his influence and experience, both on the pitch and within the dressing room, as we continue to build a team ready to challenge for the biggest honours."
Tielemans previously won the FA Cup with Leicester in 2021, scoring the winning goal in that year's final, before adding the Europa League to his honours with Villa. He arrives at Old Trafford as one of United's marquee additions of the summer transfer window.