Manchester United have paid an initial £65million, plus a potential £5million in add-ons, for the 22-year-old, with Brighton also negotiating a sell-on clause as part of the agreement. Baleba has signed a five-year contract at Old Trafford, with the option to extend for a further year.
United targeted Baleba after deciding to strengthen their midfield further following the summer arrivals of Andrey Santos from Chelsea and Youri Tielemans from Aston Villa, with the move following United's shock 2-0 defeat by Hull City in Saturday's Premier League opener, in which Santos and Tielemans made their midfield debuts.
United had first attempted to sign Baleba last year near deadline day but were put off by Brighton's valuation, which exceeded £100million.
"It is an incredible feeling to join the club of my dreams," Baleba said. "Playing at Old Trafford is always special, but the opportunity to do so as a Manchester United player will be a real honour. I cannot wait to work with Michael Carrick.
""His experience and knowledge of the game make him the perfect coach to help me to reach the highest level.
"The ambition of everyone at the club is extremely clear, the future is hugely exciting, and I am determined to play my part in delivering the success that we are capable of together."
Baleba, who joined Brighton from Lille in 2023, is currently sidelined with an ankle injury picked up during the Seagulls' pre-season training camp in Austria. He made 112 appearances for Brighton, scoring four times, and started 31 Premier League matches last season as they finished eighth to qualify for this season's Conference League.
In an emotional farewell video, Baleba reflected on a difficult campaign and thanked supporters directly.
"Last season was hard. There was a night against Aston Villa when I came off after 22 minutes. I sat in the dugout and could not lift my head up. You did not boo me. I have thought about that more than you know. You had every reason to and you did not. I will be looking for your result every week for the rest of my life. Thank you, Brighton fans."
The move takes United's summer spending to £153million as they prepare for a return to the Champions League for the first time since the 2023-24 campaign.
Captain Bruno Fernandes welcomed the signing in comments to BBC Radio 5 Live.
"A huge talent that the club has been trying to follow for a long time," Fernandes said. "Someone that can improve our team and has different qualities to what we already have in the club. The expectation is very high but also we know he is a player we need to give time to become an even better version of himself. We are here to help him settle in as quickly as possible and to help us win games."