Barca announced on Friday that they had reached an agreement with Borussia Dortmund to sign the France international for an initial fee worth €105 million.
That could rise to €145m with add-ons, which would nearly double the club’s previous club-record transfer, the €80m paid to Liverpool for Luis Suarez in 2014.
“I’m really pleased to form part of the Barca family,” the 20-year-old 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 for a good, successful year and looking forward to playing here in this stadium.
“Visca Barca, visca Catalunya.”
Barcelona have given Dembele the No. 11 shirt that was previously worn by Neymar, a baffling decision by the club as it will just heap more pressure on the 20-year-old.
But Dembele rejected the notion that he is Neymar’s replacement saying:
“There’s a big difference between Neymar and me. I’ve been a professional for two years. Neymar is one of the best in the world. I am young and I’m trying to grow each day.
“I’ve not come to be Neymar’s replacement. Barca called me to occupy a position and I have come to give my all in that position.”
Dembele becomes Barca’s fifth signing of the summer, following the arrivals of Marlon Santos, Gerard Deulofeu, Nelson Semedo and Paulinho.
Ernesto Valverde has suggested he will not be the last, with the Barca coach saying on Saturday he hopes to have “an even better team” when the transfer window closes, while sporting director Robert Fernandez said on Monday that he was aiming for two more signings this week.
Liverpool’s Philippe Coutinho remains a target and Barca hope the Premier League side will accept a fourth bid for the midfielder having seen three offers turned down so far.