PSG have also announced that Pablo Sarabia will go the other way and join the Lisbon squad on a one-season loan.
Mendes, 19, made 29 league appearances last season as part of the sports team that was crowned champion of Portugal for the first time in two decades. The defender made his senior debut for the club in December 2020.
The Portugal international becomes PSG’s sixth summer arrival in what has been a historic transfer window for the club who have also seen them capture the striker Lionel messi, Midfielder Georginio wijnaldum, defender Sergio ramos, and goalkeeper Gianluigi Donnarumma on free transfers.
Coming from the Real Madrid academy, Sarabia joined PSG in 2019 from Seville.