Chelsea confirmed that the centre-back has put pen to paper on a seven-year deal, keeping him at Stamford Bridge until 2027.
The Frenchman has set Chelsea back £70m plus add-ons, and takes the Blues’ spending for the summer to well over £200m.
On completing his signing, Fofana said: “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.”
Chelsea have also signed Marc Cucurella, Raheem Sterling, Kalidou Koulibaly, Carney Chukwuemeka, Gabriel Slonina and Cesare Casadei this summer.