The deal includes an option for Juve to make the move permanent at the end of the season, Tottenham said.
Vicario, 29, joined Spurs from Empoli in 2023 and made 117 appearances for the north London club, keeping 29 clean sheets. He was part of the side that won the Europa League in 2025, ending the club's 17-year wait for a major trophy.
The Italy international began his career at Udinese and had spells with Venezia, Perugia, Cagliari and Empoli before moving to England.
Vicario received his first senior Italy call-up in March 2024 and has five caps for his country.