The 31-year-old is the club’s second acquisition this summer after joining the England striker Raham Sterling from Manchester City.
Koulibaly has signed a four-year contract at Stamford Bridge and will step up defensive options following the departures of Antonio Rudiger and Andreas Christensen.
Koulibaly said on his new club’s official website: “I’m very happy to be here with this team at Chelsea. It’s a big team in the world and my dream has always been to play in the Premier League.
“Chelsea first came to take me in 2016 but we didn’t quite 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.”
Koulibaly spent eight years at Napoli, for whom he made 317 appearances and scored 14 goals, and penned an emotional tribute to the club’s fans in a social media post last week.
He started his career at French club Metz before spending two years at Genk in Belgium before joining Napoli in 2014.
Koulibaly has made 62 appearances for Senegal, where he currently captains.
Chelsea co-owner Todd Boehly said: “Kalidou Koulibaly is one of the world’s best defenders and we are delighted to welcome him to Chelsea.
“A great leader and exemplary team player, Kalidou brings a wealth of experience and qualities that will benefit our team and the club as a whole.”