Moussa Djenepo scored moments after coming on as a substitute to help Southampton secure their first win of the season at Brighton, who had Florin Andone sent off.
The 21-year-old Malian broke the deadlock in the second half, cutting inside from the left wing before curling a sumptuous effort past Mat Ryan and into the far corner.
Southampton sealed the points in second-half stoppage time when Nathan Redmond slid home Sofiane Boufal’s cross from close range.
Brighton had been reduced to 10 men on the half-hour mark when Andone was shown a straight red card following a high, rash challenge on Saints defender Yan Valery.
The Seagulls had looked comfortable up to that point, with Neal Maupay and Martin Montoya going close to breaking the deadlock in the first half.
Lewis Dunk thought he had given the hosts the lead seven minutes before the interval, but the goal was ruled out by VAR for offside against defender Dan Burn.
Second-half substitute Jurgen Locadia struck the post for Brighton late on, but Redmond’s late goal sealed the points for the visitors.
Seagulls fade following Andone dismissal
After narrowly avoiding relegation last season, Brighton have made a solid start to life under former Swansea City manager Graham Potter.
An eye-catching 3-0 opening-day win at Watford was followed by a 1-1 home draw with West Ham, in which summer signing Leandro Trossard delivered a man-of-the-match display.
The Belgian was heavily involved early on at the Amex Stadium, teeing up Neal Maupay whose tame effort was easily held by Southampton goalkeeper Angus Gunn.
The 24-year-old’s in-swinging cross was headed wide by Martin Montoya soon afterwards as Potter’s side continued to turn the screw.
However, the game swung the visitors’ way when Andone was dismissed following a rash challenge that left Valery writhing in agony near the half-way line.
The Seagulls responded well initially, with Lewis Dunk’s header chalked off by VAR, but Southampton capitalised on their numerical advantage in the second half.
Potter’s side almost equalised moments before Redmond’s strike when Locadia latched onto Glenn Murray’s flick-on, but the Dutchman sliced his effort against the frame of the goal as the Saints held on to inflict a first defeat on the new Brighton boss.
More to follow.