Koulibaly completes Napoli switch from Genk
The French defender's transfer was announced in May and, after his contract in Belgium expired, he has now made the switch to Serie A.
The 23-year-old has signed a five-year contract and was presented as coach Rafael Benitez's first signing of the close-season on Tuesday.
After two seasons playing in the Pro League, the defender is looking forward to testing himself in a new environment when the season begins in August.
"I'm thrilled to be playing with Napoli," he told the club's official website. "(They are) a club that has a big important project.
"I'm ready for this adventure, I am looking forward to starting work with my new team-mates."
Napoli also confirmed goalkeeper Roberto Colombo and full-back Giandomenico Mesto had agreed fresh terms at Stadio San Paulo.