The Italian has had a tough time in the Premier League, struggling to adapt to the more physical style in English football, which limited his playing time.
Giaccherini managed just 8 appearances 2014/15 season for the Black Cats, before being loaned out to Serie A side Bologna for the last campaign, where scored 7 goals in 28 league fixtures.
The 31-year-old also impressed for Italy during their EURO 2016 campaign, scoring in their 2-0 win over Belgium in the group stages.
His agent has confirmed the talks between Sunderland and Napoli, telling Radio CRC:
“We’re almost there, it’s all done for Giaccherini, the last few details are being ironed out with Sunderland. The boy is very happy. It was a choice made by him and his wife. He’s finally getting what he deserves after being underestimated for years.”
Giaccherini joined Sunderland in 2013 from Juventus, but only managed to play 32 league games across the two seasons.
The Black Cats are still unsure whether they will still be managed by Sam Allardyce, who has been heavily linked with the vacant England manager role at the FA.