In the summer of 1999, he left for Serie B side S.S.C. Napoli in a co-ownership deal. He added 10 more goals to his Serie B tally, which also helped Napoli win Serie A promotion, after finishing the season in 4th place. At the end of the season, Napoli bought Stellone's remaining registration rights, keeping him at the club. However, despite scoring on his Serie A debut the following season, in a 2–1 home defeat to Juventus on 30 September 2000, Stellone's league appearances in his first Serie A season were limited to just 3 due to injury, and as a result, Napoli were unable to avoid relegation. Stellone remained with Napoli in Serie B for the 2001–02 and 2002–03 seasons, scoring 19 goals in 51 matches, but missing out on several matches once again due to injury, however; in the summer of 2002, he was named the club's new captain. Facing financial difficulties, Napoli loaned Stellone to Serie A side Reggina during the 2003–04 season. Due to the presence of strikers David Di Michele and Emiliano Bonazzoli, Stellone struggled to find space in the team, and scored only once in 16 league appearances.