United boss Michael Carrick has overhauled his midfield since the end of last season, and Baleba will become the latest new recruit.
The 22-year-old is due to travel to Manchester this weekend to finalise the move.
He joined Brighton in 2023 from Lille and made more than 100 appearances for the Seagulls in all competitions.
Baleba was not at his best last season following interest from United, starting only 23 of Brighton's Premier League games and failing to score.
Youri Tielemans and Andrey Santos have arrived at Old Trafford in the close-season from Aston Villa and Chelsea respectively, filling the void left by Casemiro's departure and Manuel Ugarte's injury.
United, who finished third last term, start their Premier League campaign at promoted Hull on Saturday.
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