Match ends, West Ham United 0, Southampton 3.
Charlie Austin scored his fifth goal in four appearances as Southampton condemned West Ham to a fourth successive Premier League defeat.
Austin broke the deadlock in a cagey encounter with a side-foot finish following Ryan Bertrand's pull-back.
Dusan Tadic made it 2-0, collecting a pass from Austin and rounding Adrian before finding the empty net.
James Ward-Prowse's low finish consigned West Ham, who had just one shot on target, to defeat.
The facts are damning for the Hammers. They have lost five of their opening six games in a top-flight season for the first time in their history and are 18th in the table having conceded 16 goals.
Southampton, meanwhile, have won four matches in a row in all competitions and are up to eighth.
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Austin leads the way
Having selected a midfield comprising Oriol Romeu, Pierre-Emile Hojbjerg and Steven Davis, Southampton's primary aim was to contain while looking to utilise the pace and skill of Nathan Redmond and Tadic on the counter-attack.
Such a tactic is dependent on the presence of an efficient striker capable of capitalising on relatively few chances, and in Austin the Saints have just that.
The 27-year-old's strike - a well-taken first-time shot from inside the box that found the bottom corner - was the first shot on goal by either team and continued his superb recent form.
Since making his Premier League debut in August 2014, Austin has scored 21 goals - the same number as Wayne Rooney, despite playing 20 games fewer than the Manchester United and England forward.
Austin turned provider for the second goal, flicking the ball into the path of Tadic, who made up for wasting two earlier chances by skipping past the onrushing Adrian to wrap up the points.
Substitute Ward-Prowse added gloss to the display with a neat finish following Davis' driving run and cutback.
At the other end, Southampton had an equally effective protagonist in Virgil van Dijk, who made 16 clearances, including one on the line to prevent Simone Zaza giving the home side hope.
Another Hammer blow
The result and performance was far from the launch pad West Ham need in what has been a difficult season both on and off the pitch.
While defeats at Chelsea and Manchester City are perhaps unfair barometers by which to judge the Hammers, this loss following successive 4-2 defeats by Watford and West Brom marks a hugely disappointing run for a team who last season pushed for a top-four finish.
The situation has not been helped by the in-fighting between home fans during the Watford game, which contributed to an uneasy settling-in process at the London Stadium, which again was largely devoid of atmosphere.
This was another dire performance characterised by indecision and nervousness as opposed to the adventurous and fluent attacking displays that saw them score 65 league goals last season.
Poor at both ends of the pitch, the Hammers have conceded two or more goals in 13 of the past 16 league matches, dating back to March last year.
They were no better in the middle, where Dimitri Payet, who excelled for both club and country last season and at Euro 2016, cut a very frustrated figure. He was not alone.
Man of the match - Virgil van Dijk (Southampton)
What they said
West Ham boss Slaven Bilic: "They deserved to win. We didn't.
"We started well if we are talking about being behind the ball and closing the opponent down. We were not defending well as a team and in the first half we were better, compact and everything.
"But we were not happy with how we played in the last third of the pitch. We didn't penetrate enough and were unlucky in the first half.
"The second half you are trying to get back into the game and we made a big mistake. We had the ball three times and should have cleared. We tried to put them under pressure but they were always very quick in the moment of transition."
Southampton boss Claude Puel: "It was very important to win this game. Against a team without confidence it was important to press.
"We had a bad first half because we didn't keep the ball. I find a very good second half for my players with many chances and beautiful goals, which is important for confidence.
"For the future this was very good, to see the improvement from the first half to the second."
West Ham captain Mark Noble told Sky Sports: "I thought we started all right and then 'bang' we conceded a goal and to be honest we never looked back in it. If I'm honest it could have been six in the end.
"Adrian pulled off some good saves and on the bright side, I don't think it can get any worse.
"Eleven goals [conceded] in three games is laughable and it's not good enough. I think we could have kept playing until tonight and we wouldn't have scored."
The stats you need to know
- West Ham have conceded 16 goals in their opening six games of a top-flight season for the first time since 1967-68 (18).
- The Hammers have lost back-to-back home league games by a margin of two or more goals for the first time since September 2010.
- Only Tottenham (three) have kept more clean sheets than Southampton (two) in the Premier League this season.
West Ham host Middlesbrough in a 15:00 BST kick-off on Saturday, 1 October.
Southampton's next game is in the Europa League on Thursday, when they host Hapoel Beer Sheva (18:00), before returning to league action on Sunday at Leicester (14:15).
- 4NordtveitSubstituted forFletcherat 71'minutes
- 2ReidBooked at 81mins
- 5ArbeloaBooked at 72mins
- 16NobleBooked at 55minsSubstituted forFernandesat 82'minutes
- 10LanziniSubstituted forFeghouliat 45'minutes
- 27PayetBooked at 89mins
- 11ZazaBooked at 59mins
- 2Cédric SoaresSubstituted forMartinaat 89'minutes
- 6José Fonte
- 17van DijkBooked at 68mins
- 14RomeuBooked at 9mins
- 11TadicSubstituted forWard-Prowseat 80'minutes
- 10AustinSubstituted forLongat 74'minutes
- Jonathan Moss
- Shots on Target
Second Half ends, West Ham United 0, Southampton 3.
Corner, West Ham United. Conceded by Shane Long.
Goal! West Ham United 0, Southampton 3. James Ward-Prowse (Southampton) left footed shot from the centre of the box to the bottom right corner.
Shane Long (Southampton) wins a free kick in the defensive half.
Foul by Álvaro Arbeloa (West Ham United).
Dimitri Payet (West Ham United) is shown the yellow card.
Substitution, Southampton. Cuco Martina replaces Cédric Soares.
Attempt missed. Pierre-Emile Højbjerg (Southampton) right footed shot from the right side of the box misses to the left. Assisted by Shane Long.
Attempt saved. James Ward-Prowse (Southampton) right footed shot from the centre of the box is saved in the bottom right corner. Assisted by Cédric Soares.
Foul by Shane Long (Southampton).
Sofiane Feghouli (West Ham United) wins a free kick on the right wing.
Substitution, West Ham United. Edimilson Fernandes replaces Mark Noble.
Attempt missed. Pierre-Emile Højbjerg (Southampton) right footed shot from outside the box is close, but misses to the left.
Attempt blocked. Shane Long (Southampton) right footed shot from outside the box is blocked. Assisted by Nathan Redmond.
Winston Reid (West Ham United) is shown the yellow card for a bad foul.
Shane Long (Southampton) wins a free kick on the left wing.
Foul by Winston Reid (West Ham United).
Substitution, Southampton. James Ward-Prowse replaces Dusan Tadic.
Attempt missed. Winston Reid (West Ham United) right footed shot from outside the box misses to the right. Assisted by Mark Noble.