"For always. Ciao Diego," the club wrote with a blue heart emoji shortly after the news broke that the Argentine had died of a heart attack aged 60.
As a sign of mourning, Napoli changed the blue background colour of its club logo on their social media accounts to black.
Maradona spent seven years at the then-unfashionable Napoli between 1984 and 1991, during which he also lifted the 1989 UEFA Cup, Napoli's only European trophy.
Italian sports minister Vincenzo Spadafora, a native of Naples, also paid tribute to Maradona.
"The death of Maradona is terrible news. He was more than a champion, he was a football genius, an absolute champion," Spadafora said.
"In an unrepeatable season he represented the dreams and hopes of the people of my city. Naples is crying tonight."