Southampton lost at home for the second time in a row, having again been a goal up and finishing the game with ten men.
Shane Long's first goal in 23 games was instantly cancelled out by Matt Phillips' finish, before a Hal Robson-Kanu rocket sealed the three points for the Baggies.
Captain Virgil van Dijk was dismissed in the 87th minute, bringing down Salomon Rondon when he was the last man in a clumsy tackle. The Dutchman followed Nathan Redmond in seeing red, dismissed in the home defeat to Tottenham on Wednesday.
It was an interesting first half in which the home side dominated for long periods and created chances through Sofiane Boufal and Dusan Tadic. Both men showed quick footwork in wide areas to put in several low crosses that saw no teammate gambling to get on the end of them.
The Moroccan Boufal was particularly bright, dancing teasing defenders with trickery and leading an impressive break from the edge of his own box to West Brom's, Tadic having a shot blocked on that occasion.
Despite some promising play, the half lacked a clinical gloss, with Pierre-Emile Hojbjerg, Steven Davis and Maya Yoshida misplacing long passes. Tadic's set play's failed to beat the first man several times before expertly picking out Shane Long's near-post run in the 41st minute. The Irishman powered home from six yards out, scoring his first goal in 23 games.
The joy was short lived though, as West Brom levelled through Matt Phillips just 1m 36s later. Claudio Yacob found Hal Robson-Kanu, whose one-touch flick fed Phillips well, with the Englishman slotting his effort inside Forster's left post with a tidy finish.
Saints upped the anti before the break, and the pressure showed when Allan Nyom was lucky to avoid a second yellow. Long's desire saw him reach the ball first right by the byline and under the assistant referee's nose, with Nyom stamping down and making a clear foul. Hojbjerg hit the post from the resulting free kick before Nyom was replaced at half time for James McClean.
The decisive moment came early in the second half, with Phillips collecting the knock down in midfield before reversing a pass which put Robson-Kanu through on goal. The striker took the shot early, rifling a left footed drive beyond Forster's near post and into the net.
Boufal nearly replied straight away, cutting in from the left but unable to find the far corner as he did in the cup recently. It was to be the closest Saints would come to finding an equaliser as West Brom sat back and closed the game out well.
Despite Manager Claude Puel going for broke, introducing Jay Rodriguez for Hojbjerg as well as Josh Sims and James Ward-Prowse for Cuco Martina and Dusan Tadic, Southampton struggled to make an impact.
In fact, the home side were let off more than once in the second half. James Morrison's close-range header forced a save from Forster before failing to pick out a team mate in the dying moments when West Brom's break led to a three-on-one situation in the Saints' box.
It was a lesson in game management and finishing from the visitors, who took their chances and were clinical in their execution of a clear game plan. Southampton, while creative in the first half were punished for not scoring when they had the chance, and offered very little in the second period.
Infuriatingly slow build up play, including too many backwards and sideways passes were again the order of the day with sloppiness on display throughout the side in their passing, and set piece delivery which rarely beat the first defender.
Van Dijk's second yellow was fully deserved, leaving Puel with a headache as he prepares his side to face Ronald Koeman's Everton on Monday. Lose that, and the fans may start to turn with the team looking woefully short of ideas and belief.
Saints: Forster; Martina (Ward-Prowse), Van Dijk (C, R), Yoshida, McQueen; Hojbjerg (Rodriguez), Romeu, Davis; Boufal, Tadic (Sims), Long.
Subs: Taylor, Fonte, Rodriguez, Ward-Prowse, Reed, Bertrand, Sims.
West Brom: Foster; Dawson, McAuley, Evans, Nyom (McClean); Yacob, Fletcher (C); Phillips, Brunt, Chadli (Morrison); Robson-Kanu (Rondon).
Subs: Myhill, Galloway, Morrison, Gardner, Rondon, McClean, Leko.