Manchester United agreed terms with Brighton on Friday night for the Cameroon international, with an initial fee of £65 million plus a further £5 million in potential add-ons, and Brighton retaining a sell-on clause.
Baleba underwent a medical on Saturday, and journalist Fabrizio Romano confirmed on Sunday morning that all relevant documents have now been exchanged and signed between the two clubs, with the move "100 per cent confirmed" ahead of an expected official announcement. The midfielder is set to sign a five-year contract running until the summer of 2031.
It will be some time before United fans see Baleba in action, however, with the 21-year-old currently nursing an ankle ligament injury sustained during Brighton's pre-season friendly win over Strasbourg.
The issue was already expected to keep him out for the opening weeks of the season, meaning he would have missed Brighton's Premier League opener against Aston Villa regardless of the move to Old Trafford. Fellow Brighton player Yankuba Minteh, a target for Liverpool, faces an even longer absence after undergoing surgery on a right leg injury sustained against Roma and might not be fit until October or November.
It remains unclear exactly when Baleba will be ready to make his debut under Michael Carrick, with suggestions pointing to around mid-September.
United, who suffered a shock 2-0 defeat at newly promoted Hull on the opening weekend, face Ipswich and Everton before hosting Manchester City in the first derby of the season on 13 September, then travel to Fulham on 20 September. An extended three-week international break follows, with United's next fixture at home to Tottenham on 10 October.
Baleba becomes United's third new midfielder of the summer, following the £50 million signing of Andrey Santos from Chelsea and a £35 million deal with Aston Villa for Youri Tielemans, both of whom started against Hull.
The reinforcements follow Casemiro's departure at the end of his contract in July, while Manuel Ugarte, who may also leave, faces months on the sidelines after surgery on a serious knee injury suffered at the World Cup with Uruguay.