Arsenal blew the chance to go top of the Premier League as Southampton ran out impressive 4-0 winners at St Mary's.
Saints took the lead through the unlikely source of Cuco Martina, who was starting his first Premier League game for the hosts, after the defender found the corner of the goal with a superb long-range half-volley.
Shane Long added a second from close range in controversial fashion after Arsenal claimed he had fouled Laurent Koscielny in the build-up, before Jose Fonte headed home Southampton's third from a corner.
Long got the better of the Arsenal defence again in stoppage time, driving the ball under Petr Cech's legs for 4-0.
The result leaves Arsenal two points off leaders Leicester, while Saints stay 12th after their first win since November 7.
The Gunners were unchanged from their vital 2-1 win over Manchester City five days earlier, and would have leapfrogged Leicester at the summit of the Premier League with a win.
Arsenal had the first half-chance of the game as Nacho Monreal forced Maarten Stekelenburg into a low save after the ball fell to his weaker right foot 12 yards out, but Saints were ahead moments later.
Catching the ball with the outside of his right-foot 30 yards from goal, defender Martina found the bottom left-hand corner with a superb, swerving half-volley, a strike that could be a contender for goal of the season.
And Saints had a fine chance to go 2-0 up going into half-time, but Long lifted the ball over the bar after getting on the end of Sadio Mane's lofted ball having beaten Per Mertesacker for pace.
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The hosts did double their lead 10 minutes after the break, however, after Long controversially converted from close range following Mane's square ball, despite Arsenal players' protesting that the Irishman had deliberately clipped Koscielny's heels moments earlier in the build-up.
It was 3-0 with just over 20 minutes remaining as the unmarked Fonte headed home from point-blank range as Koscielny failed to deal with his opposite number's aerial presence.
Mesut Ozil's touch under Stekelenburg nearly crept over the line with 10 minutes remaining, but Southampton continued to press for another goal as Long got the better of both Koscielny and Mertesacker with his pace.
Long hit the post while one-on-one with Cech with a minute remaining, but got his second and Saints' fourth from an identical angle in stoppage time, driving the ball under the goalkeeper after picking up Dusan Tadic's through ball to round off Arsenal's humiliation.
Southampton: Stekelenburg (6), Martina (7), Van Dijk (7), Clasie (8), Fonte (7), Bertrand (7), Davis (7), Wanyama (7), Ward-Prowse (7), Mane (8), Long (9)
Subs: Tadic (6), Romeu (6), Juanmi (6)
Arsenal: Cech (5), Bellerin (5), Mertesacker (4), Koscielny (4), Monreal (5), Flamini (5), Ramsey (6), Walcott (5), Ozil (5), Campbell (5), Giroud (5)
Subs: Chambers (5), Oxlade-Chamberlain (5), Iwobi (5).
Man of the match: Shane Long