Roma announced the transfer on Sunday.
The 28-year-old Manolas joins Kalidou Koulibaly in what could become one of the top center back pairings in the Italian league.
Napoli hasn’t won Serie A since Diego Maradona led the club to its only two titles in 1987 and 1990 but coach Carlo Ancelotti is bolstering his squad in his second season with the Partenopei.
Manolas replaces Raul Albiol, who is reportedly moving to Villarreal.
Manolas’ crowning achievement with Roma came when he scored the decisive goal against Barcelona to send Roma to the Champions League semifinals in 2018.
Manolas played five seasons with Roma after starting his career in his native Greece.
Source: FOX Sport