The Slovakia international midfielder had long since agreed to stay at the club, giving his word to Aurelio De Laurentiis.
"I am very happy with this renewal," he said. "I have always said that I am very happy in Naples. It's a city that I love and I feel it's my own.
"There is great feeling with the club. There is an important project and all of us want to realise big objectives."
De Laurentiis was also pleased to tie down another of his star players.
"I am happy that Marek wanted to tie himself even more to the blue shirt. His renewal is a demonstration that Napoli wants to grow together with its talents to reinforce and to project our plan towards those objectives that our fans deserve."