"I'm really pleased to form part of the Barca family," Dembele said after signing his contract. "I have always wanted to play for Barca since I was a kid. I watched all their matches.
"I'm really happy to now be part of the best club in the world with the best players in the world. I hope to win lots of trophies and titles here. I'm hoping forward to a good year, a successful year and looking forward to playing here in this stadium."
He added: "It's an honour to play with Leo Messi. He's the best player in the history of football. I am here to learn."