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.

Coach Michel Dussuyer has also left out Newcastle's Cheick Tiote, plus Siaka Tiene and Lacina Traore for the match on 6 September.
Dussuyer says Toure is still thinking about his international future.
"At the moment he's in a period of reflection about his international future," Dussuyer said.
"I met him in London and told him I was counting on him.
"He's a big player and the captain of the team. I hope he'll continue his adventure with us."
Toure said in March that he was considering whether to continue playing for the 2015 Nations Cup winners.
Dussuyer also explained the absence of several other experienced players.
"As for Cheick Tiote we tried to get in contact with him through various means but there was no sign of life.
"I just wish he had made his position clear. Siaka Tiene still doesn't have a club,
"As for Lacina Traore and Tallo Gadji, I have to make a choice but it doesn't mean that I won't call on them in the future.
"They will just have to play well for their clubs."
The match will be played in the Nigerian city of Lagos as Sierra Leone are banned from hosting matches at home due to the Ebola crisis.
Sierra Leone lost their opening Nations Cup qualifier 1-0 to Group I opponents Sudan in June.
The group also includes 2017 Nations Cup hosts Gabon but their matches will not count towards qualification. Only the group winners advance to the finals.
Ivory Coast and Gabon drew 1-1 in June.
Ivory Coast squad:
Goalkeepers: Sylvain Gbohou (TP Mazembe, DR Congo), Ali Sangare Badra (Asec Mimosas) Made Sayouba (Stabaek, Norway)
Defenders: Serge Aurier (PSG, France), Mamadou Bagayoko (KV St Trond, Belgium), Eric Bailly (Villareal, Spain), Simon Deli (Slavia Prague, Czech Republic), Sheriff Jymoh (Athletic Adjame), Franck Kessie (Cesena, Italy), Adama Traore (Basel, Switzerland), Ousmane Viera (Rizespor, Turkey)
Midfielders: Jean Daniel Akpa Akpro (Toulouse FC, France), Ismael Diomande (St Etienne, France), Yao Serge N'Guessan (AFAD), Geoffrey Serey Die (VFB Stuttgart, Germany), Jean Michael Seri (OGC Nice, France)
Forwards: Wilfried Bony (Manchester City, England), Seydou Doumbia (CSKA Moscow, Russia), Gervinho (AS Roma, Italy), Max Gradel (Bournemouth, England), Salomon Kalou (Hertha Berlin, Germany), Giovanni Sio (Rennes, France), Thomas Toure (Bordeaux, France)
Paris St-Germain have signed Benfica winger Goncalo Guedes for £25.5m.
[NEXT_CONCEPT]
Manchester City's Yaya Toure has asked not to be included in Ivory Coast's squad for their Africa Cup of Nations qualifier against Sierra Leone.