Around 17,000 fans attended Barca's official unveiling ceremony for the 20-year-old France winger, whose switch from Borussia Dortmund was announced on Friday.
The fee agreed between the two clubs – an initial £96.8million with add-ons potentially taking it to £135.5million – makes the player the second most expensive in history.
It is eclipsed only by Neymar's £200.6million move from Barca to Paris Saint-Germain earlier this month.
Dembele was filmed by the club arriving at Barcelona's training ground on Monday and undergoing his medical. He then formally signed his five-year contract before being presented at the Nou Camp.
Dembele said: "I am very happy to be here in Barcelona. I have come here to win titles and am looking forward to integrating into the team."
He added at a later press conference: "Barcelona is the best club in the world and I am looking forward to playing with some of the best players in the world such as Luis Suarez and Lionel Messi. I will do everything possible to contribute to this team."
Dembele has enjoyed a rapid rise to prominence since breaking into the Rennes team in September 2015. He was named Ligue 1 young player of the year in 2016 and moved to Dortmund for £13million just 12 months ago.
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Meanwhile, Nice midfielder Jean-Michael Seri has reacted angrily after being denied a move to Barcelona.
The Ivory Coast international had been expected to join the Catalan giants after an impressive season as Nice maintain an unlikely Ligue 1 title push until the closing weeks.
But the move fell through and Seri - who played in both legs of the Champions League play-off defeat to Napoli but sat out Saturday's defeat to Amiens - left no doubt as to his feelings towards the directors of his current club.
The 26-year-old told El Mundo Deportivo: "Nice did not deliver on their promises, I'm very hurt. I'm not going to lie - I feel awful.
"I did not play for Nice this weekend because football should be about joy and I did not feel this. My dream of joining Barcelona has been broken and this is terrible for me."
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