Arsenal slipped to the most predictable of 4-0 defeats as Southampton ended their run of six games without a win at St Mary’s.
Three points would have placed Arsenal at the top of the Premier League after Leicester’s 1-0 defeat to Liverpool, but the Gunners were predictably poor in their first game since beating Manchester City on Monday.
The hosts were deserving of a first win since November 7, with Cuco Martina’s spectacular half-volley sending them into the break with a 1-0 lead.
Virgil van Dijk then had a header ruled out for offside at the start of the second half, before Shane Long doubled Southampton’s lead on 55 minutes.
Jose Fonte and a second from Long rounded off a quite hilarious result.
Saints remain in 12th but increase the gap to the relegation zone to seven points with victory, while Arsenal stay in second, two points off Leicester, but just one ahead of third-placed Manchester City.
Martina was the unlikely catalyst to Saints’ first win since November 7, with the full-back marking his first Premier League start with an exceptional strike with the outside of his boot from distance.
Offside calls against Steven Davis in the build-up were ignored, as was Laurent Koscielny’s appeal for a foul in the build-up to Long’s close-range strike 10 minutes after half-time.
Fonte’s header and another Long effort late on added extra gloss to the scoreline on a night when Saints’ response to recent disappointments was as marked as Arsenal’s collapse.
Southampton made four alterations for the north Londoners’ visit and changed tack following last week’s loss to Tottenham, starting at a slower, more methodical tempo as they looked to firm up at the back.
Olivier Giroud headed over and a bobbling Nacho Monreal shot were the visitors’ only attempts of note early on, with Saints’ first shot coming in the 19th minute – and what a strike it was.
With offside calls against Davis ignored, the ball eventually fell for Martina and the full-back unleashed an exquisite half-volley from 25 yards with the outside of the boot that flashed past Petr Cech.
Koeman jumped off his bench in disbelief as much as celebration and James Ward-Prowse attempted to extend that advantage, dragging wide and then curling well over.
Davis was unable to connect with a snapshot and Long saw a goal chalked off for barging Per Mertesacker in the build-up, with a booking irking the Ireland forward further.
Arsenal attempted to capitalise as Theo Walcott forced the returning Maarten Stekelenburg into a save, before the inexplicably-unmarked Southampton academy graduate headed wide from a corner.
The Gunners were beginning to knock on the door but Saints threatened to snatch a second before the break, only for Long to lift well over the crossbar after meeting Sadio Mane’s clipped pass.
Southampton did, though, find the net again five minutes after half-time, with Virgil van Dijk heading home Ward-Prowse’s free-kick under pressure.
The celebrations were short-lived thanks to the offside flag, but St Mary’s soon erupted again and there was to be no halting the revelry this time.
Long led the Saints break and played the ball out wide to Mane, continuing into the box where he met the Senegalese’s pass with a low shot past Cech.
Arsenal players were furious with referee Jon Moss for allowing the goal as Koscielny tumbled under pressure from Long in the build-up.
Things would get worse for the visitors in the 69th minute. Saints captain Fonte ghosted in at the near post to nod home a Ryan Bertrand corner, leading a number of the visiting support to head for the exits.
Dusan Tadic forced Cech into action as the home side looked to add extra embarrassment, with Long hitting the post in the 89th minute after flashing past the cumbersome Mertesacker.
Things would get worse in stoppage-time, though, as Long collected Tadic’s pass and slotted through Cech’s legs.