"It is a great pride and an immense happiness to join Paris Saint-Germain. PSG is one of the biggest clubs in the world with one of the best squads," the 22-year-old said in a statement.
Ramos made his debut for Benfica in 2020 after joining the club's academy aged 12 and scored 41 goals with 16 assists in 106 appearances for the Portuguese champions.
He made headlines in his first World Cup start last year with a hat trick to help Portugal reach the quarterfinals after taking the place of all-time leading scorer Cristiano Ronaldo.
Ramos joins new signings Marco Asensio, Milan Skriniar, Lee Kang-in, Hugo Ekitike, Lucas Hernandez and Manuel Ugarte at PSG, who sacked manager Christophe Galtier in July and replaced him with former Barcelona and Spain coach Luis Enrique.