After a tense first half, the Hammers came to life in the second period with early goals from Mark Noble and Kevin Nolan, going on to secure a crushing win over Nigel Adkins' side
Kevin Nolan and Modibo Maiga both contributed to the win with a goal each while Adam Lallana scored for the Saints.
After a tense and goalless first half, Mark Noble's free kick drifted all the way in to give the Hammers the lead just a minute after the break, with Kevin Nolan pouncing on Artur Boruc's parry two minutes later.
Adam Lallana gave the Saints a glimmer of hope after the hour but Noble's penalty and Maiga's first Premier League goal sealed a fine win for Sam Allardyce's men.
Boruc came into the side for his Southampton debut, while Mayo Yoshida got a start alongside Jos Hooiveld in central defense.
Andy Carroll continued in attack for the Hammers and Yossi Benayoun was introduced to add creativity to the midfield.
James Tomkins came closest to breaking the deadlock in the first half but his towering header was scrambled off the line.
Southampton had shipped 20 goals in seven games before this game and its defense was breached less than a minute into the second half as Noble drifted a free kick into the area that snuck past everyone and went in at the far post.
Things went from bad to worse just two minutes later as Boruc spilled Benayoun's effort straight to Nolan who tapped in his fourth of the season.
The away side rallied and created good opportunities before Lallana burst clear and smashed past Jaaskelainen.
However, the Hammers regained their two-goal cushion, Noble slamming home from the penalty spot after Carroll had been fouled in the area.
Maiga then put the icing on the cake in the closing stages, netting his first Premier League goal since moving to the Hammers in the summer.
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