The 20-year-old joins on a four-and-a-half-year deal, and Benfica could receive £6m more if he is sold on.
PSG president Nasser Al-Khelaifi said the deal for the Portugal international was completed despite rival interest from "numerous European clubs".
Earlier in January, PSG signed 23-year-old Germany forward Julian Draxler from Wolfsburg for a reported 42m euros (£35.5m).
Guedes said he hoped "to become as popular" as countryman Pauleta, the prolific striker who scored 109 goals for PSG between 2003 and 2008.
French champions PSG are currently third in the Ligue 1 table, three points behind leaders Monaco.
Guedes has played 68 times for Benfica, scoring 11 goals and providing 16 assists.
He made his professional debut with the club's reserve team, who play in Portugal's second tier, at the age of 17.
He has made two appearances for Portugal, but was not part of their Euro 2016-winning squad.
Paris St-Germain have signed Benfica winger Goncalo Guedes for £25.5m.