Saints’ six game unbeaten run in the Premier League came to an end as they were beaten 3-1 by West Ham at Upton Park.
It all looked to be going their way as Maya Yoshida headed Saints into an early lead.
However, despite dominating possession for large swathes of the game, goals from Matt Jarvis, Carlton Cole and Kevin Nolan gave the Hammers victory.
Kevin Nolan inspired West Ham to fourth successive Premier League victory as Saints were left to rue missed opportunities at Upton Park.
The east Londoners' rise shows no sign of abating, with their 100 per cent record in February taking them from the relegation zone to the dizzy heights of 10th.
West Ham had to ride their luck at times against a Southampton side unbeaten in six league matches, but impressively came from behind to secure a 3-1 victory on home soil.
This is the first time the Hammers have won four consecutive top-flight matches since the end of the 2006/07 season and a run which surely makes Sam Allardyce a shoo-in for manager of the month, just weeks after his future looked in jeopardy.
A tight defence has been key to that upturn, but hopes of a fifth successive top-flight clean sheet evaporated after just eight minutes, when Mark Clattenburg, in charge of a Saints match for the first time since the club complained about his conduct, awarded a free-kick for a foul on Adam Lallana, from which Maya Yoshida nodded the visitors ahead.
Saints were looking impressive, but the match turned on its head in a crazy three-minute spell in the first half, with Matt Jarvis drawing the Hammers level after meeting a fine long ball by Nolan.
Calls for offside were waved away and Mauricio Pochettino's side were soon behind, with Carlton Cole following up his header off the bar to fire past Artur Boruc.
West Ham target Rickie Lambert hit a post as half-time approached and Southampton continued to press for a leveller in a second half they dominated.
However, they were made to rue their flurry of missed cha nces as Nolan acrobatically turned home his sixth goal in nine matches to secure West Ham three points.
This was Southampton's first league defeat since New Year's Day - a result that looked unlikely at the start.
Lallana spearheaded the bright opening spell and was adjudged by Clattenburg to have been taken down by James Tomkins, with Yoshida rising highest to direct Steven Davis' free-kick past Adrian.
It was the first league goal conceded by West Ham since January 18 and, in truth, it could have come earlier was it not for Guy Demel's timely early challenge on Jay Rodriguez.
Saints were moving the ball around with confidence, but the Hammers were beginning to settle and Nolan prodded goalward after Demel crossed from the right.
While the east Londoners improved, few could have predicted the way the match would swing during a frantic three-minutes spell.
Out of nothing, Nolan showed impressive vision to launch the ball into the path of Jarvis, who had got behind the Saints defence. He controlled well and slotted his finish through the on-rushing Boruc's legs.
Calls for offside against the West Ham winger came to nothing and the stunned visitors were soon picking the ball out of their net again thanks to Cole, who reacted quickly after seeing a powerful header rattle the bar to crash home.
It was quite a turnaround and the hosts' new-found confidence was epitomised by even James Collins trying an effort from distance.
Saints eventually composed themselves after a sloppy spell and came close to drawing level in the 32nd minute, with Lambert beating Collins in the air to head a Jack Cork cross on to a post.
The visitors continued to pour forward, with Lallana, Calum Chambers and Luke Shaw all attempting to force a goal before the break.
Stewart Downing saw a cross just evade Nolan as the first half came to a close, with Southampton returning after half-time rejuvenated.
The visitors claimed to no avail that Collins handled in the box eight minutes after the restart before Shaw stung the palms of Adrian.
The West Ham goalkeeper also produced a fine save to deny Morgan Schneiderlin, unaware the offside flag had been raised, as Saints continued in the ascendancy, with Collins heading over at the other end in a rare forage forward.
The Welsh centre-back was soon back defending his own goal - attempting to at any rate. Shaw showed fantastic pace to dart past Collins on the left, only for Lallana to just fail to turn the ball goalward.
Substitute Gaston Ramirez was the next to leave the Hammers defender for dead, flicking the ball through his legs and continuing to lash just over.
Lallana turned wide under pressure after a misplaced Demel header and those missed chances proved costly as Nolan acrobatically turned home in the 71st minute.
With Saints' centre-backs doubling up on Cole from a Downing cross, the ball ran dropped kindly for Nolan to score.
The Hammers captain left the field to a standing ovation soon after, replaced by Winston Reid.
Fellow substitute Mohamed Diame fired against the crossbar with a fine long-range effort, but it was Saints on top for the remainder of the match.
Adrian did well to tip a low Lallana shot around a post and Yoshida saw a header blocked, while Rodriguez and Ramirez also came close before the final whistle, but it was not to be for Saints.