The 31-year-old Italy international was in demand after an impressive Euro 2016 and he joins Maurizio Sarri’s side for a reported £2million fee.
He has called time on his Black Cats career after three years. The attacker joined the Wearsiders – then managed by compatriot Paolo Di Canio – from Juventus in July 2013 but failed to cement a first-team place at the Stadium of Light.
Giaccherini made 43 appearances for the Black Cats, scoring five goals, and he duly returned to Italy last season on loan with Bologna.
His impressive performances with the northern outfit – and subsequently with Italy last month – have convinced last year’s Scudetto runners-up Napoli to bring him to Stadio San Paolo for a cut-price fee, with his transfer being announced on sscnapoli.it on Saturday afternoon.