The 19-year-old's performances for Sporting Lisbon had led to suggestions that the likes of Manchester City and Manchester United were keen to sign him during the summer transfer window.
Sporting had seemingly won the battle to keep hold of the Portuguese, but it was announced on August 31 that PSG had agreed to sign the defender on loan for the 2021-22 campaign.
According to Italian journalist Fabrizio Romano, PSG will pay €7m (£6m) to loan the full-back and will then have the option of signing him permanently for €40m (£34.3m) next summer.
Mendes has made 47 appearances for Sporting, contributing one goal and three assists, while he travelled with the Portugal squad to this summer's European Championship.
Gianluigi Donnarumma, Sergio Ramos, Georginio Wijnaldum, Lionel Messi, Danilo Pereira and Achraf Hakimi all joined Mauricio Pochettino's side during a busy summer transfer window.