Match ends, Southampton 4, Arsenal 0.
Arsenal missed the chance to go top of the Premier League as they were thrashed by Southampton at St Mary's.
Saints full-back Cuco Martina marked his first league start with a stunning goal, curling in from 30 yards out.
Arsenal's Olivier Giroud and Theo Walcott missed free headers, before Shane Long finished from close range and Jose Fonte headed in the third.
Striker Long struck the post with three minutes remaining, but managed to add a fourth through the legs of Petr Cech.
It could have been even worse for the visitors. Virgil van Dijk had a header ruled out for offside, while Dusan Tadic's dipping strike was pushed away by Cech.
Super Saints but Gunners sinners
Arsenal had lost just one of their previous 11 league games and recorded an impressive victory over Manchester City on Monday but Arsene Wenger's side could not back that performance up with victory on the south coast.
Having seen leaders Leicester beaten by Liverpool earlier in the day, the Gunners would have gone top with a win but were resoundingly beaten by a rampant Southampton side.
Playmaker Mesut Ozil - who has received plenty of plaudits for his impressive performances this season - was kept quiet by the Southampton midfield, while former Saint Walcott posed little threat, attempting only 20 passes before being substituted.
Going into the game, Arsenal had conceded the joint-fewest number of goals (14) in the league this season and their second-half capitulation will be a big worry to Wenger.
Arsenal need to win games against teams in the bottom half of the table if they are to take the title, but did not come close to capitalising on the points dropped by Claudio Ranieri's side.
'Bully' Wanyama gives Arsenal no respite
The pressure was starting to grow on Southampton boss Ronald Koeman after five winless games left them just four points above the relegation zone.
But they were marshalled brilliantly in the middle of the park by the superb Victor Wanyama, who made a match-high six tackles and 10 interceptions, often bullying the opposition players.
The opening goal was stunning. There seemed to be no danger when defender Per Mertesacker headed the ball away, and Martina steadied himself before unleashing an outside-of-the-foot drive which started wide of the post before swerving into the net.
Arsenal posed little threat and the hosts added to their tally with a slick counter-attacking move.
There was a hint of controversy in it though, as Long and Laurent Koscielny came together in the build-up, leaving the Frenchman on the ground, allowing Long to stride forward unchallenged and stroke in from Sadio Mane's pass.
Defender Fonte added a bullet header and Long grabbed his second to complete a comprehensive victory for Southampton who managed their first clean sheet in six games.
Man of the match - Shane Long
What they said:
Southampton manager Ronald Koeman: "You don't expect a 4-0 win against Arsenal. Of course we have a good football team, but we have struggled in recent weeks. But the second half was perfect. It was a tough night for Arsenal, we caused them a lot of problems. It was an amazing result."
Arsenal manager Arsene Wenger: "We lost too many challenges and that explains why we lost the game. But that's one aspect, so well done to them.
"The second aspect is that on the first three goals we were really unlucky with the decision of the referee. The first goal was offside, the second goal was a foul and the third goal was a goal kick, so if you're a bit below par and on top of that you have the first three goals against you in the decision-making then it's even more difficult."
The stats you need to know
- This was Arsenal's heaviest Premier League defeat since a 6-0 away loss at Chelsea in March 2014
- Arsenal have conceded five Premier League goals from outside the box in 2015-16 so far; two more than in the whole of the 2014-15 season
- Southampton winger Sadio Mane has six assists in 18 Premier League appearances this season - he had only three in 30 league games last season
- Shane Long has scored five and assisted two more goals in his last seven Premier League starts for Southampton
It's a quick turnaround for both sides. Southampton travel to West Ham on Monday (17:30 GMT), while Arsenal will look to get back to winning ways when they host Bournemouth on the same day (also 17:30 GMT).
- 6José Fonte
- 17van Dijk
- 4ClasieSubstituted forRomeuat 67'minutes
- 16Ward-ProwseSubstituted forTadicat 71'minutes
- 10ManéSubstituted forJuanmiat 81'minutes
- 7LongBooked at 34mins
- 20FlaminiSubstituted forChambersat 73'minutes
- 28CampbellSubstituted forOxlade-Chamberlainat 64'minutes
- 14WalcottSubstituted forIwobiat 78'minutes
- 54Reine Adelaide
- Jonathan Moss
- Shots on Target
Second Half ends, Southampton 4, Arsenal 0.
Corner, Southampton. Conceded by Laurent Koscielny.
Attempt blocked. Dusan Tadic (Southampton) left footed shot from the centre of the box is blocked. Assisted by Shane Long.
Goal! Southampton 4, Arsenal 0. Shane Long (Southampton) right footed shot from the centre of the box to the bottom right corner. Assisted by Dusan Tadic with a through ball.
Attempt blocked. Juanmi (Southampton) left footed shot from the left side of the box is blocked. Assisted by Oriol Romeu.
Foul by Laurent Koscielny (Arsenal).
Shane Long (Southampton) wins a free kick in the attacking half.
Attempt blocked. Oriol Romeu (Southampton) right footed shot from the centre of the box is blocked. Assisted by Dusan Tadic.
Shane Long (Southampton) hits the left post with a right footed shot from the right side of the box. Assisted by Dusan Tadic.
Offside, Southampton. Cuco Martina tries a through ball, but Shane Long is caught offside.
Foul by Alex Oxlade-Chamberlain (Arsenal).
Juanmi (Southampton) wins a free kick in the defensive half.
Foul by Calum Chambers (Arsenal).
Oriol Romeu (Southampton) wins a free kick in the defensive half.
Attempt saved. Olivier Giroud (Arsenal) header from the centre of the box is saved in the top centre of the goal. Assisted by Alex Oxlade-Chamberlain with a cross.
Substitution, Southampton. Juanmi replaces Sadio Mané.
Offside, Southampton. Victor Wanyama tries a through ball, but Sadio Mané is caught offside.
Olivier Giroud (Arsenal) wins a free kick on the right wing.
Foul by Virgil van Dijk (Southampton).