The former Spain international, who was without a club following his summer release from Tottenham, has penned a contract with Carlo Ancelotti's side after garnering much interest from elsewhere.
Tottenham had been linked with a move to take him back to north London in order to provide cover for Harry Kane.
Manchester United and Fiorentina were also understood to be interested in making a late move to bring him in, but Partenopei have won the race for his signature, despite Ancelotti previously being coy on links.
No official details have been disclosed but it has been reported that the 34-year-old has signed a deal worth around 2.5million euros per year at the Stadio San Pablo.
Llorente is no stranger to playing football in Serie A, having represented Juventus for two years from 2013 to 2015.
In his two years, he scored 27 goals in 92 appearances but lost his first-team place and joined Sevilla, where he remained for just one year.
He then joined Swansea and scored 15 goals in 33 Premier League appearances before earning a move to Spurs where served as back up to Kane.