West Ham ensured that Ivan Juric's Southampton tenure got off to the worst possible start, with Jarrod Bowen's 60th-minute goal sealing all three points for under-fire Julen Lopetegui and Co.
The England forward scored the only goal of the game with a close-range prod off a Niclas Fullkrug flick-on, after his team-mate Lukasz Fabianski was taken off on a stretcher earlier in the game.
The defeat leaves the hosts still stranded at the bottom of the table, eight points from safety with Juric facing a massive task if he is to keep the Saints in the division.