Match ends, Southampton 1, Cardiff City 2.
Southampton manager Ralph Hasenhuttl says his side have "no chance of staying in the league" if they continue to perform as they did in Saturday's 2-1 home defeat by fellow strugglers Cardiff.
Kenneth Zohore struck in the third minute of stoppage time to earn the visitors all three points on an emotional afternoon for the Welsh side, who continue to mourn the death of record signing Emiliano Sala.
Jack Stephens looked to have rescued a point for the hosts moments earlier when he tapped Charlie Austin's flick-on past Neil Etheridge at the far post.
Sol Bamba had opened the scoring for Neil Warnock's side, beating Alex McCarthy from close range with the visitors' first effort on target.
Burnley's victory at Brighton in Saturday's late game means Southampton slip into the bottom three for the first time since early January.
"Losing in that way is a brutal thing for me," said Hasenhuttl. "It's unbelievable how easy we gave those points away.
"We did it last week [against Burnley] and the same again today. It can feel like a win if you draw like that, and then you give it away. That's stupid and unbelievable for me.
"But it doesn't change anything. We have to fight until the end, as I was saying in December.
"It's very narrow, very close, but that was clear in December and it doesn't change in February."
The win lifts Cardiff out of the relegation zone and into 15th place.
Cardiff emerge victorious on emotional day
The match at St Mary's was preceded by a minute's silence in memory of Sala, whose body was formally identified on Thursday after being recovered from the wreckage of the plane that crashed over the English Channel last month.
For the third game in succession, Cardiff players and staff wore yellow daffodils in honour of the Argentine striker, while the visitors also warmed up in T-shirts bearing Sala's image.
Warnock said before the game that the struggling Bluebirds must be prepared for "13 cup finals" as they attempt to come to terms with the death of the former Nantes player, and the 70-year-old will have been proud of his side's resolute defending against their relegation rivals.
Etheridge was the busier of the two goalkeepers, but Southampton were mainly restricted to shots from long range as the visitors stood firm.
Cardiff had barely threatened at the other end in the opening 65 minutes, but Bamba's goal against the run of play - a smart finish at the far post - sent the away supporters behind McCarthy's goal into raptures.
Their hopes of claiming three points appeared over when Stephens struck in second-half stoppage time, but Zohore's effort - his first of the season - ensured Cardiff recorded consecutive top-flight victories for the first time since April 1962.
Having accused his players of "giving two points away" in last weekend's draw at Burnley, Ralph Hasenhuttl will be wondering how his side failed to claim at least a point here.
The hosts enjoyed 67% possession at St Mary's, but despite finding the target with seven of their 14 attempts they struggled to test Etheridge in the Cardiff goal.
In a first-half short on clear-cut opportunities, Nathan Redmond fired a low effort into the side-netting before Pierre-Emile Hojbjerg shot straight at Etheridge from the edge of the penalty area.
They went within a whisker of an equaliser when Lee Peltier cleared Charlie Austin's effort off the line, moments after Yan Valery had been thwarted by Etheridge when one-on-one.
Few would have argued with the result had the game ended after Stephens' late equaliser, but the Saints failed to hang on as Zohore condemned them to their first league defeat since December.
Having picked up just two points from three games against fellow strugglers Crystal Palace, Burnley and Cardiff, Southampton face a daunting trip to Arsenal in their next fixture.
Man of the match - Neil Etheridge (Cardiff)
Cardiff strike late again - the stats
- Southampton lost for the first time in six Premier League games (W2 D3).
- Cardiff have won consecutive top-flight games for the first time since April 1962.
- Southampton have kept just one clean sheet in their past 16 Premier League games (0-0 v Chelsea in January).
- Cardiff have scored three winners after 90 minutes in the Premier League this season, a joint-high figure along with Wolves.
- Southampton have both scored (12) and conceded (14) in each of their past eight home league games.
- Cardiff have failed to score a first-half goal in a divisional-high 20 Premier League games this season.
- Cardiff's opener from Bamba on 69 minutes came courtesy of their first shot on target.
'We wanted to do it for Emiliano' - what the managers said
Southampton manager Ralph Hasenhuttl: "They shot three times on our goal and scored two goals. We had not many chances but more than Cardiff and scored only once.
"If you score an equaliser in added time, you have to take a minimum of one point. It would help us a lot for the mentality, the atmosphere and the feeling for the future.
"What we did now was to roll a big stone in our way and to remove that will take a lot of time."
Cardiff manager Neil Warnock:
"They don't know when they're beaten. It was a deflation to concede so late - everybody thought it was the key to them going and getting the winning goal."
"I thought we were super after that. I'm really pleased for Ken and Victor [Camarasa] - both the subs helped for the winning goal."
"We wanted to do it for Emiliano and I'm really proud the lads have done him justice."
Cardiff host Watford in the Premier League on Friday 22 February (19:45 GMT), while Southampton travel to Arsenal two days later (14:05 GMT).
- 4VestergaardSubstituted forElyounoussiat 72'minutes
- 43ValerySubstituted forGallagherat 83'minutes
- 14RomeuBooked at 90mins
- 16Ward-ProwseBooked at 45mins
- 22RedmondBooked at 89mins
- 7LongSubstituted forAustinat 72'minutes
- 5Ecuele Manga
- 22BambaBooked at 70mins
- 17GunnarssonBooked at 61minsSubstituted forBacunaat 62'minutes
- 14De Cordova-ReidSubstituted forZohoreat 62'minutesBooked at 90mins
- 29NiasseSubstituted forCamarasaat 83'minutes
- Martin Atkinson
- Shots on Target
Second Half ends, Southampton 1, Cardiff City 2.
Oriol Romeu (Southampton) is shown the yellow card for a bad foul.
Foul by Oriol Romeu (Southampton).
Harry Arter (Cardiff City) wins a free kick in the defensive half.
Kenneth Zohore (Cardiff City) is shown the yellow card for excessive celebration.
Goal! Southampton 1, Cardiff City 2. Kenneth Zohore (Cardiff City) left footed shot from the centre of the box to the bottom right corner. Assisted by Víctor Camarasa.
Attempt missed. Harry Arter (Cardiff City) left footed shot from the right side of the box misses to the left. Assisted by Callum Paterson with a headed pass.
Offside, Cardiff City. Kenneth Zohore tries a through ball, but Sol Bamba is caught offside.
Foul by Mohamed Elyounoussi (Southampton).
Harry Arter (Cardiff City) wins a free kick on the right wing.
Goal! Southampton 1, Cardiff City 1. Jack Stephens (Southampton) right footed shot from very close range to the centre of the goal. Assisted by Charlie Austin following a corner.
Attempt missed. Charlie Austin (Southampton) header from the centre of the box misses to the right. Assisted by James Ward-Prowse with a cross following a corner.
Corner, Southampton. Conceded by Joe Bennett.
Nathan Redmond (Southampton) is shown the yellow card.
Sam Gallagher (Southampton) wins a free kick in the defensive half.
Foul by Lee Peltier (Cardiff City).
Foul by Mohamed Elyounoussi (Southampton).
Kenneth Zohore (Cardiff City) wins a free kick on the right wing.
Attempt missed. Charlie Austin (Southampton) header from the centre of the box is high and wide to the left. Assisted by James Ward-Prowse with a cross.