The released Tottenham striker, 34, signed a two-year deal with the Azzurri, but could've ended up at St Etienne this week.
France Football says ASSE made a serious pitch to Llorente to bring him to France.
Llorente 34, said yesterday: "I am very happy to start this new adventure with the Napoli team! Surely great moments are waiting for us to live together.
"I will do my best to achieve many important goals. I thank the fans for the affection you have shown me. Forza Napoli."