Southampton converted a last-minute penalty to give Mauricio Pellegrino his first Premier League win of the season.
Pellegrino handed a Premier League debut to new signing Mario Lemina, who had signed for Saints earlier that week, and he replaced James Ward-Prowse in the starting eleven.
Saints got off to a flying start, as Nathan Redmond and Manolo Gabbiadini produced some fine counter-attacking football, opening a gap for Redmond to play through Gabbiadini, who shrugged off former Saint José Fonte and slotted the ball into the far corner, breaking a nine-hour goal drought at home for the Saints.
West Ham’s day got even worse when Marco Arnautovic got a straight red for elbowing Jack Stephens, so the Hammers were down to ten men.
Redmond, Steven Davis and Gabbiadini had a few more chances to go ahead before the break, but the second came from the penalty spot, as Davis was brought down by Fonte again, giving Dusan Tadic a chance to double Saints lead.
The Serbian struck straight down the middle, hitting the feet of a diving Joe Hart and spinning into the net, meaning that the Saints had a two-goal cushion going into the break.
This was too good to be true, however, as Michail Antonio struck a low shot at keeper Fraser Forster, but Javier Hernandez was first to react, as he tapped the ball into a half-empty net.
The visitors got an equaliser early in the second half, as an Aaron Cresswell cross found the head of Diafra Sakho, it collided with Forster, came off the bar, and Javier Hernandez was there once again to tap it in.
In added time, a James Ward-Prowse cross was lifted over to Maya Yoshida but was brought down in the box by Pablo Zabaleta, awarding a last-gasp chance for the Saints, and it was the substitute Charlie Austin who stood up to the spot.
The striker slotted the ball into the bottom left corner, winning the game for Saints.
Here are five things we learned about the game.
