The 18-year-old winger, a French youth international, has signed a five-year contract with the German giants.
Dembele has been a revelation for Rennes this season, scoring 12 goals and adding five assists. He was being tracked by a number of top European clubs.
Borussia Dortmund signs top talent @Dembouz - read all about it here: https://t.co/GHURKOgMse pic.twitter.com/zTBjQxv05q
— Borussia Dortmund (@BVB) May 12, 2016