The 18-year-old winger signed a five-year deal to join Borussia Dortmund from Rennes on Thursday, with the German Bundesliga club describing him as "one of Europe's most sought after talents".
Dembele has been a revelation for Rennes this season, scoring 12 goals and adding five assists since making his debut in November.
The Bundesliga side did not give a figure for the deal, but German media said the fee was in the region of 15 million euros ($17.1m).
British media had earlier reported that Liverpool were ready to make a blockbuster offer, but the player had his heart set on Dortmund.
France under-21's take on Italy in Venise on June 2 in a memorial match for an Italian, Valeria Solesin, who died in the Paris terror attacks in November.
Her death moved a lot of people in Italy, with one magazine, l'Espresso, going so far as to name her their 'person of the year' for 2015.