The one-time Italy international left Juve on a permanent deal in 2018 and has since become a regular at Udinese.
But the Serie A champion confirmed the signing of Mandragora on Sunday (AEST), although the 23-year-old will stay in Udinese.
The fee will be paid across two seasons, with Mandragora – who has signed a contract with Juve until 2025 – staying on loan at Udinese for 2020-2021.
Udinese also holds an option to extend that loan through 2021-2022.
While the loan is free, Juventus said Udinese may receive bonuses up to €6 million ($9.9 Million).
Mandragora has been sidelined since June after suffering a knee injury.