The Spanish forward left the Tottenham Hotspur Stadium at the end of last season having failed to sign a new deal under Mauricio Pochettino.
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The 34-year-old has moved back to Serie A as a free agent having previously spent two years at Juventus.
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The Spaniard had been linked with Manchester United this summer after the sales of Alexis Sanchez and Romelu Lukaku.
Tottenham Hotspur’s Fernando Llorente and Victor Wanyama (right) celebrate after the final whistle of the Champions League quarter-final
Image credit: PA Sport
Llorente will soon return to England, however, after Napoli were drawn in the same Champions League group as Liverpool, the conquerers of Spurs in the Madrid final last May.
The striker scored 13 goals in 66 appearances across two years with the club as Harry Kane's deputy.
Napoli were beaten 4-3 against Juventus on Saturday, so they will hope Llorente hits the ground running.
The club's owner Aurelio De Laurentiis tweeted a picture of his new striker signing a contract on Monday morning.
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