The centre-back has expressed his delight after finalising the switch following the expiry of his contract with Genk, hailing his new club as "big and important"
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 Belgian 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 have also confirmed that goalkeeper Roberto Colombo and full-back Giandomenico Mesto have agreed fresh terms at Stadio San Paulo.