Fofana has signed a seven-year contract at Stamford Bridge after protracted negotiations with the Foxes.
Thomas Tuchel's side will pay £70m for Fofana's services, with a further £5m coming as add-ons.
Fofana only signed a new contract until 2027 with Leicester back in March after returning from a long-term knee injury that restricted him to just 12 appearances last season.
The 21-year-old was one of the revelations of the 2020/2021 season following a summer move from Saint Etienne as he made 38 appearances across all competitions to help Leicester win the FA Cup and finish fifth in the Premier League.
He started Leicester's first two games of the season before being overlooked for the next two as his future looked uncertain.
Fofana told Chelsea's official website: ‘The two last days have been really big days for me and I’m very happy. I trained this morning with the team and it’s a dream for me. I’m very excited to start playing games for the fans and the club.
‘I’m here to win trophies – the Champions League, Premier League, FA Cup, Carabao Cup, everything. I came here to win and the club is built to win trophies so I’m here to continue that.’
Chelsea chairman Todd Boehly added: 'Wesley is a hugely promising defender who has already proven his quality in the Premier League at a young age.
'We are delighted we were able to bring one of Europe’s most exciting talents to Chelsea and further strengthen that area of our squad, both for this season and many more to come.’
