The 26-year-old becomes new Azzurri head coach Rafael Benitez's first signing at the Stadio San Paolo following two successful seasons with Eredivisie club PSV Eindhoven.
A statement published on sscnapoli.it read: "Napoli announce the signing of Dries Mertens, a Belgian winger who arrives from PSV Eindhoven. Mertens has signed a five-year contract."
The right-footed left winger played his first competitive games on loan from Gent with Eendracht Aalsts, where he scored four times before moving to AGOVV and notching 30 goals in three years.
A move to Utrecht followed and 16 goals in two seasons earned him a move to PSV in 2011. Mertens would score more than a goal every two games for the Rood-witten with a haul of 37 strikes from 62 appearances.
His time at the Philips Stadion also saw him lift the Dutch Cup and Dutch Supercup in 2012 and, having made his international debut in February 2011, Mertens has scored twice in 19 outings with Belgium.