The French champions are obliged to buy Ramos for €65m at the end of the 2023/2024 season with a further €15m in add-ons.
Ramos came through Benfica's youth ranks and broke into the first team in the 2019/2020 season.
He made a total of 106 appearances for Benfica and scored 41 goals.
Last season was his most prolific at the Estadio Da Luz, hitting the back of the net 27 times in 47 games across all competitions.
The 22-year-old earned international recognition at last year's FIFA World Cup in Qatar when he scored a hat-trick against Switzerland in the round of 16.
Speaking on the team's official website, he said: "I'm very proud and happy to join Paris Saint-Germain.
"PSG are one of the biggest clubs in the world with one of the best squads."
PSG president Nasser Al-Khelaifi added: "We are very happy to welcome Goncalo as an exciting new striker in our PSG family.
"He is a fantastic international player who is young, hungry and fights for the team. It is these kinds of players who are the future of our great institution."
PSG lost the services of Lionel Messi at the end of last season while Neymar and Kylian Mbappe have uncertain futures.
