Trossard’s goal was just the second shot on target for the visitors, but the piece of quality their play had been missing to that point as they inflicted a first defeat of the season on their newly-promoted hosts.
The Belgian forward picked up the ball on the edge of the box, skipped away from two defenders and curled his shot into the bottom corner to stun the home fans, who had watched their side waste a number of opportunities to win the game.
The best fell to Bryan Mbeumo, who was teed up on a number of occasions by the brilliant Ivan Toney, but could not find a first goal of the Premier League season.
Toney spotted Brighton goalkeeper Robert Sanchez off his line early but his audacious effort from 40 yards sailed narrowly over the bar. He then set up Mbeumo 10 yards out but the French forward fired wildly over the bar, before the same two combined for a similar result from further out.
It was Mbeumo getting into the good positions when Brentford might have wished it was Toney, and after the latter set up his team mate yet again, this time the shot from the edge of the area curled wide of the post as another chance went begging.
Brighton climbed to third in the table.