The Blues will pay an initial £53million to Southampton for their newest midfield recruit, with a further £5m potentially coming by way of add-ons.
Lavia has signed a seven-year contract at Stamford Bridge through 2030 to take the club’s spending on midfielders alone this week to £170m, following the British record £115m capture of Moises Caicedo from Brighton on Monday.
The 19-year-old also had strong interest from Liverpool but, like Caicedo, opted to move to west London over Anfield.
Chelsea first bid for the player only weeks after his move from Manchester City to Southampton last summer and co-director of recruitment and talent Joe Shields is thought to have played an instrumental role in the deal.
Shields brought Lavia to the south coast upon leaving City last year and has since taken up a role at Chelsea alongside co-sporting directors Laurence Stewart and Paul Winstanley.
Former Anderlecht youngster Lavia, who has one senior international cap for Belgium to his name so far, will wear the No45 shirt at Chelsea this season, previously held by the likes of Nathaniel Chalobah and Fabio Borini.
It is the same number that he had at Southampton, where he made 34 appearances across all competitions last term as the Saints were relegated from the Premier League. Lavia could be in line for a swift Chelsea debut in Sunday’s top-flight derby against West Ham at the London Stadium.