Santi Cazorla’s stoppage-time penalty gave Arsenal a dramatic victory over a determined Southampton side.
Saints took the lead when Petr Cech tipped a Dusan Tadic free-kick onto the underside of the bar but the ball bounced in off the Gunners keeper.
Laurent Koscielny equalised with a spectacular overhead kick after Cazorla’s corner was not cleared.
The hosts toiled in attack but Cazorla won it for them after Jose Fonte was judged to have fouled Olivier Giroud.
The result was harsh on Southampton, who were well-organised at the back and dangerous when they broke forward.
Saints remain winless under new manager Claude Puel, while Arsenal are five points behind Premier League leaders Manchester City, who beat Manchester United earlier on Saturday.
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Reaction from all of Saturday’s Premier League matches
How did Arsenal’s new boys do?
Arsenal boss Arsene Wenger gave debuts to both his recent signings, £35m centre-back Shkodran Mustafi and £17m forward Lucas Perez.
A World Cup winner with Germany, Mustafi instantly looked comfortable in his new surroundings, having more touches (94) and playing more passes (86) than anyone else on the pitch, frequently stepping up into the Saints half when he was on the ball.
But Spaniard Perez had a much quieter debut, failing to have a shot of any kind in the first half. His only effort at goal after the break was blocked and he was taken off after 63 mostly ineffective minutes.
Wenger threw on Giroud, Alexis Sanchez and, eventually, Alex Iwobu too, but his side continued to struggle in front of goal until Fonte tangled with France striker Giroud and referee Robert Madley pointed to the spot.
Southampton are in the bottom three after failing to win any of their first four games of the season, but they were unfortunate not to pick up at least a point in north London.
Shane Long was left on the bench until half-time but had three good chances in the second half.
Long wasted his best chance when he fired well wide after Tadic’s superb flick had left him with only Cech to beat.
The Republic of Ireland striker also headed over the bar when unmarked from a free-kick and did not get enough on his follow-up shot after Cech had parried a Pierre Hojberg strike.
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