Wijnaldum, 30, has agreed a three-year deal with PSG after deciding to run down his contract at Anfield, which expires at the end of June.
"Signing for Paris Saint-Germain is a new challenge for me," Wijnaldum said. "I'm joining one of the best squads in Europe and I want to bring all my desire and commitment to this ambitious project. Paris Saint-Germain have proved how good they are in recent years and I am convinced that together, with our supporters, we can go even further and higher."
The Netherlands international had previously reached an agreement to join Barca. His agents met with the Catalan club's president Joan Laporta at the end of May and a medical had been pencilled in.
Barca's interest in Wijnaldum dates back to last summer, when Ronald Koeman made him a target after being appointed as the club's coach.
However, as first reported by ESPN on Friday, PSG made a late play to hijack Barca's move for Wijnaldum.
Sources close to the negotiations told ESPN that the Ligue 1 side were willing to double the terms agreed with Barca, while a phone call from coach Mauricio Pochettino also helped persuade Wijnaldum to change his mind.
Bayern Munich also held talks with Wijnaldum, who informed the German club and Barca last weekend that he had decided to sign for PSG.
Wijnaldum, who is preparing for the European Championship with Netherlands, played for Feyenoord and PSV Eindhoven before making the move to England with Newcastle United in 2016 for around €15 million.
After Newcastle were relegated from the Premier League a year later, he joined Liverpool on a five-year deal for a fee of €25m.
He helped Liverpool win the Champions League in 2019, famously scoring twice in the semifinal win over Barca, and was part of the side which won the Premier League a year later.
In total, he made 237 appearances for Liverpool, scoring 22 goals.