The award is given to the best player under the age of 21 in Europe and is voted for by a panel of journalists from across the continent.
The 18-year-old shone at Monaco last season and has scored four goals and has four assists in 11 games for Paris Saint-Germain this season having moved in the summer on an initial loan deal.
The striker, who already has eight international caps for France, won with 291 votes beating second-placed Dembele, who secured 149 votes.
Wayne Rooney (2004) and Raheem Sterling (2014) have previously won the award which is given to the player who has been in the best form over the previous calendar year.