Southampton punished two errors by Arsenal goalkeeper Wojciech Szczesny to stay fourth in the Premier League.
Szczesny raced from his goal in the first half to allow Sadio Mane to curl into an empty net from a narrow angle.
Saints' keeper Fraser Forster superbly saved from Alexis Sanchez before Szczesny kicked a loose ball while on the floor to Dusan Tadic, who poked in.
Southampton could have scored more, with Graziano Pelle's shot cleared from the line, as Arsenal stay fifth.
Saints' manager Ronald Koeman suggested his side could finish above Arsenal this week, a feat which seemed unlikely in the summer as almost £100m of players left.
A run of four league defeats from late November issued a reality check for the south coast club but dreams of a top-four finish once again seem realistic with 10 points taken from the last 12 on offer.
Without the suspended Morgan Schneiderlin - a reported target for many teams in January - Koeman's side were solid all over the pitch, none more so than goalkeeper Forster, who saved brilliantly from Santi Cazorla and Sanchez either side of Mane's opener.
By contrast, Szczesny made errors which proved the difference between the teams.
First, the Poland keeper raced from his goal to confront Mane, but the Senegal striker - soon heading for the African Cup of Nations - curled in his third goal in as many games with a measured finish from the byline.
The Arsenal keeper then superbly denied James Ward-Prowse from 12 yards before the break, but panicked when grounded in his six-yard box before the hour mark and kicked a loose ball straight to Tadic, who scored his second Premier League goal.
It was the seventh goal Arsenal have conceded from an error this season - only Everton, with 11, have more.
The Gunners nearly added to that tally when Szczesny was sold short by a backpass and Mathieu Debuchy was left to clear Pelle's prod off the line.
The Italy striker had more shots than any other Premier League player in the first half of the season and came close to adding to his eight league goals when he hit the post twice.
Arsenal, who suffered a first defeat in five league outings, had a strong case to be playing 10-men with a quarter remaining when Florin Gardos felled Sanchez as he raced in on goal.
Referee Craig Pawson issued only a yellow and although Arsenal introduced Theo Walcott for the first time since November, they barely threatened to reduce the arrears.
Credit: The BBC
