The Argentinian goalkeeper will join the Naples outfit this summer, remaining on loan at Catania until the end of the season.
Napoli were keen to acquire a goalkeeper to replace Pepe Reina, who is expected to leave the Naples outfit this summer.
Reina, who is on loan at Napoli from Liverpool, has been strongly linked with a move to the Primera Liga, with Barcelona among the clubs keen to acquire his services.
Napoli's statement read: "Napoli have signed Mariano Andujar.
"The Argentinian goalkeeper joins Napoli on a permanent transfer from Catania but will play on loan at the Sicilian outfit until June, 2014."
Andujar joined Catania in the summer of 2009 from home club Estudiantes.
The 30-year-old has made 13 league appearances for Catania this season after falling down the pecking order, with goallkeeper Alberto Frison currently starting for the Sicilian outfit.