PSG were overwhelming favourites coming into this tie against opponents who had never taken part in European competition before this season, and there seems little prospect of Brest recovering from this loss in next week's second leg.
Vitinha opened the scoring for PSG at the Stade de Roudourou in Guingamp with a penalty just before the midway point in the first half, and Dembele swept in a second on the stroke of half-time before netting again on 66 minutes.
The French international forward now has 23 goals for the season, including 18 in 11 appearances over the last two months. Indeed he has 10 goals in just the last four games, a run which includes a hat-trick against Brest in Ligue 1 only 10 days ago.
He is irresistible just now, and Luis Enrique's side are unbeatable at the moment against French opponents, against whom they have not lost since last May.
The teams will meet again in the return leg of this knockout phase play-off tie next Wednesday in Paris, with the winners advancing to a last-16 showdown with either Liverpool or Barcelona. Assuming they make it through, PSG will feel they are capable of beating either of those sides on current form.