Dembele has agreed a five-year contract running until the summer of 2028.
The 26-year-old is returning to France for the first time since 2016 when he left Stade Rennes to sign for Borussia Dortmund.
Dembele won three Spanish La Liga titles, two Copa del Reys and the Spanish Super Cup twice during his time with the Blaugrana.
His Barcelona career was however often blighted by injury issues which restricted him to 185 appearances and 40 goals across all competitions.
Dembele still had one year remaining on his contract but PSG nonetheless opted to activate his release clause.
"I'm delighted to be joining Paris Saint-Germain and can't wait to play for my new club," Dembele told club media.
"I hope I can continue to grow here and make all the club's fans proud."
President Nasser Al-Khelaifi added: "We are delighted to welcome Ousmane Dembele to Paris Saint-Germain where he will be an important and committed player for our club.
"The passion and determination shown by Ousmane as he has signed for PSG is fantastic and perfectly corresponds to the attitude required of all of our players. We are proud to be able to bring another French World Cup winner to Paris Saint-Germain as we enter a new great era for our club.“
