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 now 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.