In an official press release, the Serie A side said the two clubs had reached a definitive agreement.
In bocca al lupo, Stephy!
— JuventusFC Youth (@JuventusFCYouth) June 30, 2020
The sale will give Juventus a capital gain of €4m on 22-year-old Mavididi.
The €6.3m fee is slightly higher than the €5m reported by L’Equipe on Monday.
His performances in Ligue 1 led to Montpellier wanting to buy him outright, which they now have.
Although mainly signed for the reserve team, he did make one senior appearance for Juve, against SPAL in April 2019.
That appearance made him the first English player since David Platt in 1992 to play for the Old Lady.