Dembele penned a five-year contract at the Camp Nou alongside club president Josep Maria Bartomeu on Monday afternoon.
The 20-year-old becomes the most expensive player in Barca’s history and the second most expensive in world football, eclipsed only by Neymar ‘s recent £200m move to Paris Saint-Germain.
The France international, whose release clause stands at 400m euros (£369.6m), told the official Barca website: “I’m very happy to be here.
“It has always been my dream to be at Barcelona and now I’m here I’m very happy to have achieved my dream. It is the best club in the world with the best players in the world.”
Upwards of 17,000 fans were reported to have gathered at the ground on Monday to catch a glimpse of the player, who inherits Neymar’s number 11 shirt.
The former Rennes player’s last appearance for Dortmund came in the German Super Cup against Bayern Munich on August 5.