The 19-year-old signed for PSG for a €7 million loan fee, with the French club also having an option to buy for €40 million, with Pablo Sarabia going the other way on a season-long loan, as a dramatic transfer window came to an end.
So who exactly is Nuno Mendes?
Mendes joined Sporting's youth academy at the age of 10, and due to the 2019/20 season being prolonged because of the COVID-19 pandemic, Mendes was handed the opportunity to play for the first-team in June 2020, at just 17.
He came on as a substitute for experienced left-back Marcos Acuna, but the Argentine was then sold to Sevilla at the end of the season, leaving Mendes as Sporting's first-choice left-back despite being only 18.
Mendes became one of the team's most important players last season, making 35 appearances in all competitions, playing either at left-back or left-midfield.
Ruben Amorim's team flourished in Portugal, winning the Primeira Liga, the Taca da Liga and also the Supertaca Candido de Oliveira.
Mendes was integral to their title wins, with him also being named in the Primeira Liga Team of the Year, being the youngest player in the squad.
His impressive performances earned him a contract extension with Sporting, increasing his release clause from €45 million to €70 million.
Mendes made his senior Portugal cap in 2021, and he even earned a spot in the squad for UEFA Euro 2020, and while he did not play a game, it was still a great experience for him.
As a result of his solidity and maturity, PSG went in for him this summer, and while the likes of Manchester City were also interested, he eventually joined the Ligue 1 club on a season-long loan for a fee of €7 million, with an option to make the deal permanent for €40 million.
Mendes is a very adventurous full-back, wanting to get forward whenever he can, and he will fit in with PSG's play style, often being dominant in possesion in Ligue 1 matches.
Last season, Mendes averaged 1.5 dribbles per league game, also making 1.1 key passes and completing 1.4 crosses per game, registering a goal and an assist (stats via Whoscored).
However, he is also very resolute defensively, making 1.7 tackles per game and 2.1 interceptions per game, as Sporting conceded just 20 goals in 34 games, having the best defence in the league.
At the age that Mendes is at, he needs regular game time, and going to PSG, who have both Juan Bernat and Layvin Kurzawa playing in the same position as him might be a concern, yet the pair are both out with relatively lengthy injuries, and so if he can start well with the French club, then he could cement his place in the starting eleven.