The 31-year-old winger joined the Black Cats from Juventus in July 2013 but failed to cement a first-team place at the Stadium of Light and he duly returned to Italy on a season-long loan with Bologna for the 2015-16 campaign.
His impressive performances with the northern outfit - and subsequently with Italy at Euro 2016 - have convinced last year's Scudetto runners-up Napoli to recruit him for a reported £2million fee, with his transfer being announced on sscnapoli.it on Saturday afternoon.
Giaccherini enjoyed a reasonable start to life on Wearside, scoring four league goals and setting up eight more in his debut season under Paolo Di Canio and then Gus Poyet, but a series of injuries kept him on the sidelines for most of the campaign that followed.
He would miss a month of his year on loan with Bologna through injury but recovered to hit seven goals as the Rossoblu finished 14th in the Italian top flight.
Having missed out on a place at the 2014 World Cup he made the most of his inclusion in Antonio Conte's Azzurri selection for the summer's European Championship, helping Italy reach the quarter-finals, where he was among those who managed to score a spot-kick in a penalty shootout defeat to Germany.
Reports had linked Arezzo-born Giaccherini with Chelsea - now under the supervision of Conte, his manager at Juve - but instead ambitious Napoli have moved to bolster their own attacking options with last term's 36-goal striker Gonzalo Higuain looking increasingly likely to leave the Stadio San Paolo.
Upon his arrival in Naples, Giaccherini told sscnapoli.it: "I've joined a prestigious club and I'm very happy to pull on this shirt.
"I can't wait for my adventure with Napoli to begin
Playing at San Paolo will be fantastic."
Source : PA
Source: PA