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The Serie A leader, which is 12 points clear at the summit, have signed the 27-year-old until the end of the season, with an option to make the move permanent.
Gollini's move came as Salvatore Sirigu left Napoli to join Fiorentina in something of a swap transfer.
Fiorentina loaned Gollini from Atalanta at the start of the season, but he made just nine appearances for the Viola, conceding eight goals.
Gollini spent last season on loan at Tottenham, for whom he played 10 times.
He will be tasked with challenging Alex Meret, who has been an ever-present for Napoli in the league this term, conceding 14 goals in 19 games.