Match ends, Southampton 1, West Ham United 0.
Southampton earned their fourth win in five matches, despite a third red card of the season for midfielder Victor Wanyama.
Saints started strongly and took the lead when Maya Yoshida smashed home Sadio Mane's centre.
Wanyama received a straight red from referee Mark Clattenburg for diving in on West Ham midfielder Dimitri Payet.
The hosts survived a late scare when defender Cedric sliced a clearance narrowly past his own post.
Striker Emmanuel Emenike - a January arrival on loan from Fenerbahce - made his debut for the Hammers, but he missed the opportunity to earn a point for his side when he headed wide from Enner Valencia's cross.
The victory lifts Southampton to seventh in the Premier League table, just one place and two points behind their opponents.
Wanyama's moment of madness
West Ham will be disappointed not to take anything from the game after playing the last 37 minutes with an extra man.
Wanyama - in his third season at St Mary's - is one of Southampton's senior players and can be a highly effective operator at the base of midfield, but his appetite for a tackle can also be his undoing.
Having already been dismissed against Bournemouth and Norwich this season, Wanyama became the first player since Wes Brown in 2013-14 to receive three red cards in the same season.
In truth, it was perhaps a case of the player's reputation preceding him, as Wanyama's challenge didn't appear to make significant contact with Payet, but Mark Clattenburg - whose tally of five red cards this season is the second-highest among referees - decided that the tackle was worthy of a straight red.
Saints defence stands firm again
That Southampton were able to see out the remainder of the match with 10 men owed much to another excellent performance from their back four, plus goalkeeper Fraser Forster.
Since Forster returned from patellar tendon surgery last month, the team has recorded five clean sheets in a row - roughly seven and a half hours of football without conceding.
It wasn't one of Forster's busier evenings - though he did show his class with a brilliant reflex stop from a Winston Reid header - as centre-backs Virgil van Dijk and Jose Fonte dominated Hammers strikers Enner Valencia and substitutes Emenike and Andy Carroll.
Missed opportunity for wasteful Hammers
For West Ham, this was a missed opportunity to overhaul Manchester United in fifth place - and the Hammers will rue their profligacy in front of goal.
Slaven Bilic's side had 18 shots - but only found the target with two.
Enner Valencia had been in excellent form, with four goals in his previous four games, but here the Ecuadorian striker was uncharacteristically wayward, shanking an effort wide from the edge of the area when well placed.
Man of the match - Maya Yoshida
What they said
Southampton manager Ronald Koeman: "I'm very pleased with the final result. It was a difficult game from the start, more difficult after the red card to Victor Wanyama.
"But even with one less on the pitch we had the best chances to score a second. We showed very good defensive composure, good defending. I don't remember one big chance [for West Ham]."
On Wanyama's sending off: "You can't do that tackle. It's a hard tackle. You give the referee the possibility to give the red card."
West Ham manager Slaven Bilic: "First I would like to congratulate Southampton. They were very good at defending in the box. They deserve this.
"We did not create enough. Our crosses were quite poor, they defended really well. When you lose a game like this, we are gutted.
"For me it was a red card. When that second leg comes... it was a reckless challenge."
The stats you need to know
- Two of Maya Yoshida's three Premier League goals have been against West Ham United.
- Victor Wanyama is the seventh player to be sent off three times in the same Premier League season, joining Vinnie Jones, David Batty, Slaven Bilic, Craig Short, Frank Queudrue and Wes Brown.
- There has been a Premier League high of 10 red cards in West Ham games this season (four for them, six for the opposition).
- Fraser Forster's clean-sheet percentage is the best of any goalkeeper with 30 or more Premier League appearances, with 18 clean sheets from his 35 games (51.4%).
Southampton visit Swansea next Saturday, while West Ham travel to Norwich.
- 17van Dijk
- 6José Fonte
- 2Cédric Soares
- 12WanyamaBooked at 54mins
- 4ClasieBooked at 83minsSubstituted forWard-Prowseat 90+3'minutes
- 10ManéSubstituted forRomeuat 56'minutes
- 19PellèSubstituted forAustinat 72'minutes
- 5TomkinsSubstituted forByramat 61'minutesBooked at 87mins
- 30AntonioSubstituted forEmenikeat 73'minutes
- 20MosesSubstituted forCarrollat 61'minutes
- 11E ValenciaBooked at 67mins
- Mark Clattenburg
- Shots on Target
Second Half ends, Southampton 1, West Ham United 0.
Attempt blocked. James Collins (West Ham United) left footed shot from the centre of the box is blocked.
Substitution, Southampton. James Ward-Prowse replaces Jordy Clasie.
Corner, West Ham United. Conceded by Cédric Soares.
Attempt missed. Enner Valencia (West Ham United) right footed shot from outside the box is too high from a direct free kick.
Emmanuel Emenike (West Ham United) wins a free kick in the attacking half.
Foul by Oriol Romeu (Southampton).
Attempt missed. Charlie Austin (Southampton) right footed shot from the centre of the box is close, but misses to the right. Assisted by Oriol Romeu.
Sam Byram (West Ham United) is shown the yellow card for a bad foul.
Foul by Sam Byram (West Ham United).
Ryan Bertrand (Southampton) wins a free kick on the left wing.
Offside, West Ham United. Dimitri Payet tries a through ball, but Sam Byram is caught offside.
Alexandre Song (West Ham United) wins a free kick in the defensive half.
Foul by Shane Long (Southampton).
Attempt missed. Enner Valencia (West Ham United) right footed shot from the right side of the box is high and wide to the right. Assisted by Dimitri Payet.
Jordy Clasie (Southampton) is shown the yellow card.
Foul by Sam Byram (West Ham United).
Charlie Austin (Southampton) wins a free kick in the defensive half.
Attempt blocked. Andy Carroll (West Ham United) left footed shot from the centre of the box is blocked. Assisted by Alexandre Song with a headed pass.