Fenerbahce have confirmed the transfer of Elif Elmas to Napoli in an official statement released on the Istanbul stock exchange.
Napoli will pay an initial €16m fee which could rise to €19m in total with bonuses – including a €1.5m sell-on fee clause.
The other €1.5m fee will be paid depending on the number of appearances Elmas makes.
Fenerbahce and Napoli were locked in talks for weeks over the transfer of the 19-year-old.
Yellow Canaries sporting director Damien Comolli rejected several offers from the Serie A side before finally reaching an agreement a source close to the club revealed to Turkish-Football.
Fenerbahce signed Elmas for just €300,000 from Rabotnicki in the summer of 2017.
The attacking midfielder went onto score four goals in 40 appearances for the Yellow Canaries in all competitions.
Elmas joins a growing list of young players to leave the Super Lig for top-five leagues.
Nineteen-year-old Ozan Kabak was signed by Stuttgart on an €11m move from Galatasaray in January before being transferred to Schalke for €16m over the summer.
Twenty-one-year-old Merih Demiral joined Sassuolo from Alanyaspor in January before being snapped up by Juventus on an €18m move in July.