Match ends, Arsenal 2, Southampton 1.
Arsenal 2-1 Southampton
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.
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.
Saints unlucky to leave empty-handed
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.
Man of the match - Arsenal's Santi Cazorla
Arsenal begin their Champions League campaign on Tuesday (19:45 BST kick-off), with a trip to play French champions Paris St-Germain, before another away game, at Hull, in the Premier League next Saturday (15:00 BST).
Southampton are also on European duty, at home to Sparta Prague in the Europa League on Thursday (20:05 BST), before they host Swansea on Sunday (14:15 BST).
The stats you need to know
- Arsenal scored with both of their shots on target in this match, from 17 shots in total.
- Arsenal are unbeaten in their past 22 home league games against Southampton (W16 D6).
- The Gunners have already won more Premier League penalties in 2016-17 (three) than they did in the whole of 2015-16 (two).
- Koscielny became the fifth Arsenal player to score a Premier League goal on their birthday (after Paul Merson, Robert Pires, Yossi Benayoun and Cazorla).
- Saints scored their first Premier League goal at the Emirates since a 1-6 defeat in September 2012.
- Cech scored his second Premier League own goal, and first since December 2009 (for Chelsea against Everton).
- Cazorla's past seven Premier League goals for Arsenal have been penalties.
- 18MonrealBooked at 16mins
- 19CazorlaBooked at 26mins
- 14WalcottSubstituted forIwobiat 75'minutes
- 15Oxlade-ChamberlainSubstituted forSánchezat 62'minutes
- 9PérezSubstituted forGiroudat 62'minutes
- 1ForsterBooked at 54mins
- 2Cédric Soares
- 6José FonteBooked at 90mins
- 17van DijkBooked at 90mins
- 21BertrandBooked at 90mins
- 4ClasieSubstituted forHøjbjergat 64'minutesBooked at 86mins
- 11TadicSubstituted forWard-Prowseat 80'minutes
- 9RodriguezSubstituted forLongat 45'minutes
- Robert Madley
- Shots on Target
Second Half ends, Arsenal 2, Southampton 1.
Attempt saved. Shane Long (Southampton) header from the left side of the box is saved in the centre of the goal. Assisted by Cédric Soares with a cross.
Goal! Arsenal 2, Southampton 1. Santiago Cazorla (Arsenal) converts the penalty with a right footed shot to the centre of the goal.
Ryan Bertrand (Southampton) is shown the yellow card.
Virgil van Dijk (Southampton) is shown the yellow card.
Delay over. They are ready to continue.
Delay in match Laurent Koscielny (Arsenal) because of an injury.
José Fonte (Southampton) is shown the yellow card.
Penalty Arsenal. Olivier Giroud draws a foul in the penalty area.
Penalty conceded by José Fonte (Southampton) after a foul in the penalty area.
Attempt blocked. Shkodran Mustafi (Arsenal) header from the centre of the box is blocked. Assisted by Mesut Özil with a cross.
Corner, Arsenal. Conceded by Cédric Soares.
Foul by Alexis Sánchez (Arsenal).
Oriol Romeu (Southampton) wins a free kick in the defensive half.
Pierre-Emile Højbjerg (Southampton) is shown the yellow card for a bad foul.
Alex Iwobi (Arsenal) wins a free kick in the defensive half.
Foul by Pierre-Emile Højbjerg (Southampton).
Attempt missed. Olivier Giroud (Arsenal) header from the centre of the box is close, but misses to the left. Assisted by Santiago Cazorla.
Francis Coquelin (Arsenal) wins a free kick in the defensive half.
Foul by Virgil van Dijk (Southampton).
Corner, Southampton. Conceded by Petr Cech.
Attempt saved. Shane Long (Southampton) left footed shot from the centre of the box is saved in the centre of the goal.
Attempt saved. Pierre-Emile Højbjerg (Southampton) right footed shot from outside the box is saved in the bottom left corner. Assisted by James Ward-Prowse.
Alexis Sánchez (Arsenal) wins a free kick in the attacking half.
Foul by James Ward-Prowse (Southampton).
Substitution, Southampton. James Ward-Prowse replaces Dusan Tadic.
Attempt missed. Olivier Giroud (Arsenal) header from the centre of the box is high and wide to the right. Assisted by Santiago Cazorla with a cross following a corner.
Corner, Arsenal. Conceded by Cédric Soares.
Attempt blocked. Nacho Monreal (Arsenal) left footed shot from the centre of the box is blocked. Assisted by Alex Iwobi.
Foul by Olivier Giroud (Arsenal).
Pierre-Emile Højbjerg (Southampton) wins a free kick in the defensive half.
Substitution, Arsenal. Alex Iwobi replaces Theo Walcott.
Attempt missed. Shane Long (Southampton) header from the centre of the box is too high. Assisted by Nathan Redmond with a cross following a set piece situation.
Foul by Theo Walcott (Arsenal).
Virgil van Dijk (Southampton) wins a free kick on the left wing.
Attempt missed. Nathan Redmond (Southampton) right footed shot from outside the box is too high. Assisted by Pierre-Emile Højbjerg.
Attempt blocked. Alexis Sánchez (Arsenal) left footed shot from the left side of the box is blocked. Assisted by Olivier Giroud.
Offside, Southampton. Shane Long tries a through ball, but Nathan Redmond is caught offside.
Attempt missed. Alexis Sánchez (Arsenal) right footed shot from the left side of the box is just a bit too high. Assisted by Mesut Özil.