Barcelona said they had reached an agreement with PSG on Dembele's transfer for 50.4 million euros.
Last year, he extended his contract with the Spanish champions until June 2024 after his previous deal expired. He won three LaLiga titles and two Spanish Cups in six years.
"I'm delighted to be joining Paris St Germain and can't wait to play for my new club," Dembele told the club's website. "I hope I can continue to grow here."
Dembele started his playing career at French side Stade Rennais, making his debut in the 2015–16 season before moving to Bundesliga club Borussia Dortmund.
He signed with Barca for 105 million euros plus add-ons on a five-year deal in 2017 and scored 40 goals along with 43 assists in 185 appearances.
The forward has 37 international caps and was part of France teams that won the World Cup in 2018 and reached the final in December.
Dembele joins new signings Marco Asensio, Milan Skriniar, Lee Kang-in, Hugo Ekitike, Lucas Hernandez and Manuel Ugarte at PSG. The club sacked manager Christophe Galtier in July and replaced the Frenchman with former Barca and Spain coach Luis Enrique.
PSG will aim to win a record-extending 12th Ligue 1 title this season and challenge for the Champions League - a trophy that has eluded them despite their domestic dominance. They begin their title defence at home to Lorient on Sunday.
"We are delighted to welcome Ousmane Dembele to Paris St Germain where he will be an important and committed player for our club," PSG chairman Nasser Al-Khelaifi said.
"We are proud to be able to bring another French World Cup winner to Paris St Germain as we enter a new great era for our club."
