Sources told ESPN on Wednesday that Chelsea were close to signing the Senegal captain in a deal worth €40 million.
Chelsea's attempts to sign Juventus defender Matthijs de Ligt have so far been unsuccessful, while sources told ESPN Sevilla's Jules Kounde is of long-standing interest having come close to agreeing a deal last summer.
Introducing our second signing of the summer! 🔵#KoulibalyIsChelsea— Chelsea FC (@ChelseaFC) July 16, 2022
"I'm very happy to be here with this team at Chelsea," Koulibaly told Chelsea's official website. "It's a big team in the world and my dream was always to play in the Premier League.
"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."
Koulibaly, 31, joined Napoli in 2014 and has made 317 appearances for the club. He previously had spells with Belgian side Genk and French team Metz.
Information from ESPN's James Olley was used in this report.