Brighton ended Brentford's unbeaten start to the season as Leandro Trossard scored the only goal of the game in the 90th minute in west London.
The Belgian curled into the corner on the stroke of full-time to punish Thomas Frank's side for a lack of cutting edge and inflict a first defeat at home since Valentine's Day.
The newly promoted Bees had created the better of the chances, but Bryan Mbeumo was guilty of failing to make the most of them and they were hit with a sucker-punch late on.
Brighton's deadline-day signing Marc Cucurella was handed a full debut and was introduced to life in England with a flailing arm from fellow Spaniard Sergi Canos, which earned an early yellow card.
The Brentford fans, who had been treated to a memorable opening-day home win over Arsenal, were almost on their feet celebrating inside 10 minutes when Ivan Toney tried his luck from 40 yards, but his chipped effort sailed just over the crossbar, much to the relief of away goalkeeper Robert Sanchez.
Danny Welbeck had a better opportunity for the away side soon after and yet his weak effort was too close to David Raya before the hosts got into their groove.
A neat free-kick routine saw Mbeumo cross into the area for Toney, but his header landed on the roof of the net.
The Bees' front duo were causing plenty of problems and a wonderful touch by Toney from Raya's long-range pass saw him get away down the left before he cut back for his strike partner.
Frenchman Mbeumo curled over, however, and did the same again 60 seconds later at the midway point of the first period after an exquisite dummy by Toney, who seemed buoyed by his first top-flight goal at Aston Villa.
Brighton boss Graham Potter had another problem to solve when Adam Webster went down with a muscle injury. Eventually the centre-back limped off to be replaced by Jakub Moder, which saw Joel Veltman moved inside from his right wing-back role.
It did remain goalless at the break, though, with Mbeumo again wasteful after more excellent link-up play with Toney, rifling wide from 16 yards before Vitaly Janelt fired straight at Sanchez as Frank's side were unable to make their first-half dominance count.
The Seagulls' frustration with referee Graham Scott had started to boil over at the end of the opening 45, but it was the patience of the official which ran out when he booked Veltman and Shane Duffy in quick succession.
While Brentford had looked the more likely to break the deadlock, former striker Neal Maupay had the first chance of the second period when he dragged an effort wide on the turn.
Shandon Baptiste had suffered an injury-hit 2020-21 campaign but failed to mark his first Premier League start with a goal when he curled over for the hosts with 20 minutes left as both managers rolled the dice with changes.
Brighton substitute Alexis Mac Allister saw an effort deflect wide in what appeared the final opportunity until he found Trossard outside the area and the Belgian worked a yard of space away from Pontus Jansson to rifle home.
It sparked wild celebrations in the away end, where Seagulls chairman Tony Bloom was present, and helped the visitors make it three wins out of four in the league this season.
