The Senegal international has signed a four-year contract at Stamford Bridge to become Chelsea's second signing of the summer after Raheem Sterling.
The 31-year-old comes with a wealth of experience from Serie A after starring for Napoli for the last eight seasons.
The Senegal international, who captained his country to Africa Cup of Nations glory earlier this year, made 314 appearances for Napoli in his time at the club and won the Coppa Italia and Supercoppa Italiana.
Koulibaly had one year remaining on his Napoli contract and they did not want to lose him for free next summer.
Koulibaly's arrival at Chelsea will help compensate for the loss of Antonio Rudiger and Andreas Christensen to Real Madrid and Barcelona respectively on free transfers.
"I'm very happy to be here with this team at Chelsea. It's a big team in the world and my dream was always to play in the Premier League," Koulibaly told the official Chelsea website.
"Chelsea came first to get me in 2016 but we didn't make it. Now when they came to me I accepted it because they really wanted me to come to the Premier League to play for them.
"When I spoke to my good friends Edou [Edouard Mendy] and Jorginho they made my choice easier so I'm really happy to be with you today.
"I want to thank the fans because I saw a lot of them in London and on the plane everybody was happy for me to be here. So I want to thank them and I hope the season will be really good and we will give some good moments to the fans."
Blues owner Todd Boehly added: "Kalidou Koulibaly is one of the world's elite defenders and we are delighted to be welcoming him to Chelsea. A great leader and an exemplary team player, Kalidou brings a wealth of experience and attributes that will benefit our squad and the club as a whole."
