The 31-year-old international from Senegal played 317 times for the Italian team after moving from Genk to Napoli in 2014.
Following the free transfers of defenders Andreas Christensen to Barcelona and Antonio Rudiger to Real Madrid, he joins Stamford Bridge.
Koulibaly said, “I’m thrilled to be at Chelsea with this squad.
“It’s a big squad globally, and I’ve always wanted to compete in the Premier League. In 2016, Chelsea tried to woo me initially, but we were unsuccessful. They genuinely wanted me to join the Premier League and play for them, so when they approached me, I agreed.
My decision was made more straightforward when I talked to my close pals, Edou [Mendy] and Jorginho.
Senegal’s 2021 Africa Cup of Nations champion, Koulibaly, made his senior debut in September 2015 and has since amassed 62 caps for the nation.
After Raheem Sterling signed for the club in a £50 million agreement, he became Chelsea’s second acquisition since the Todd Boehly-led takeover.
Boehly declared: “One of the best defenders in the world, Kalidou Koulibaly, is joining Chelsea with great excitement.
“Kalidou offers a wealth of experience and traits that will enhance our squad and the club as a whole. He is a wonderful leader and an exceptional team player.”