JORDY Clasie’s first Premier League goal gave Saints a vital victory in their quest for a top-eight finish.
The Dutchman’s outstanding 25th-minute strike, a 20-yarder that flew into the top corner, proved to be the winner at the Hawthorns.
It closed the gap on eighth-placed West Brom to four points and completed an excellent eight days for Saints, who have two games in hand on the Baggies.
Claude Puel’s side has taken seven points from their last three games and back-to-back wins - and three wins from their last four away games - mean a fourth successive top-eight finish now looks very achievable.
Without Manolo Gabbiadini and with Shane Long struggling to make an impact, there was no end product from Saints in the first 25 minutes, despite attractive approach play from Dusan Tadic, Nathan Redmond and Ryan Bertrand.
Saints were relieved the offside flag was raised when Phillips’ pass behind the back four was curled into the feet of the lurking Salomon Rondon, who had screwed wide an early shot and been dispossessed by the impressive Maya Yoshida.
Fraser Forster had an excellent afternoon and was forced into the first save in the 20th minute when, after chesting the ball down 25 yards out, the recalled James Morrison drove a half-volleyed left-footer goalwards.
Forster was equal to it, diving low to his left to palm it away.
Yoshida slid into dispossess Rondon after a clever pass from Nacer Chadli and West Brom won a corner when Bertrand blocked Craig Dawson’s cross. Fortunately, the near-post header flashed across goal.
Saints countered but Redmond’s cross was inaccurate and Matt Phillips responded with an electric run along the Saints byline, winning a corner that was overhit.
Then Clasie produced the game’s first moment of real quality, a first-time 20-yarder that fairly flew in to the top right corner.
In the absence of Steven Davis (injured) and Oriol Romeu (suspended), the Dutchman was playing at the base of the Saints midfield alongside Pierre-Emile Hojbjerg.
Whe he popped up on the edge of the WBA area he drove the ball, from Tadic’s lay-off, past the stationary Ben Foster.
Long played the ball inside to Tadic, who showed great composure to turn and pass for the unmarked Clasie to rifle in his first-ever Premier League goal.
The strike was very similar to his only other goal for Saints, in the 2-0 EFL Cup win against Arsenal at the Emirates in November, and was a fine way to mark his first start since January.
Hitherto, the game had had an end-of-season feel in glorious sunshine. But with West Brom within catching distance in eighth position, Saints had plenty to play for.
Encouraged by their goal, they continued to attack.
Tadic drove a free-kick in the Baggies wall after the dangerous Redmond was tripped by Allan Nyom.
Saints argued the case for another dead ball when former Baggies forward Shane Long appeared to be brought down on the edge of the area after being put through by Tadic.
Replays showed any contact from Jonny Evans to be minimal, as the home fans heaped abuse on Long.
Shortly afterwards, Forster’s athleticism was needed to maintain Saints’ lead Nacer Chadli’s right-footed outswinger was headed across goal by Rondon from six yards out at the far post.
But Forster was equal to it, diving to his left before clawing the ball away one-handed.
At the other end, Long failed to hit the target with a diving header from Redmond’s cross.
Half-time: West Brom 0 Saints 1
Phillips was booked at the start of the second half for a foul on Clasie and Redmond jinked into the area before his shot was blocked.
Foster came out of his area to head clear from Long, following a lobbed pass from Tadic, who half- volleyed a shot narrowly wide before West Brom responded with the substitution of Gareth McAuley, who was replaced by Chris Brunt at left-back.
Foster was equal to a Bertrand cross at his near post.
Long waved to the West Brom fans as they continued to vilify him and left the pitch to chants of ‘he’s going to cry in a minute’ after a clash of heads with Craig Dawson, before being booed as he came back on.
Phillips was replaced by James McLean just past the hour, shortly after Jake Livermore had driven WBA’s first shot of the second half over the bar.
There were appeals for a penalty when Forster dived at the feet of Rondon. With 25 minutes left, Darren Fletcher was replaced by Hal Robson-Kanu, who scored the winning goal when West Brom won at St Mary’s on New Year’s Eve.
Redmond was Saints’ biggest goal threat in the first 20 minutes of the second half.
After firing one shot at Foster he did well to volley Cedric Saores’ cross on target, but Foster was equal to it at his near post. Saints were grateful Chadli fired straight at Forster when well placed to replicate Clasie’s goal.
The 24,697 crowd (including 2,289 Saints supporters) was the biggest for a Premier League match at the Hawthorns since Manchester United’s visit four months ago and they made their presence felt as West Brom went in search of an equaliser.
Forster did well as subs McLean and Robson-Kanu threatened to make an impact before Cedric, having been booked for a foul, conceded a corner against McLean.
Forster punched clear and Yoshida, who was excellent against Rondon all afternoon, thwarted him twice more.
Hojbjerg received treatment after going down under a challenge from Livermore but it was Clasie who went off before play resumed, replaced by Harrison Reed with six minutes left.
Forster ensured that Clasie’s goal was the winner with a save-of-the-season contender from Dawson’s flying volley in injury-time.
Saints breathed a sigh of relief when Morrison fired the rebound over the bar and Forster ensured a well-deserved clean sheet with a brilliant tip over the bar from Evans.
SAINTS: Forster, Cedric, Yoshida, Stephens, Bertrand, Clasie (Reed 84), Hojbjerg, Ward-Prowse (McQueen 71), Tadic, Redmond (Sims 71), Long. SUBS: Hassen, Caceres, Pied, Rodriguez.
WEST BROM: Foster, Dawson, McAuley (Brunt 53), Evans, Nyom, Fletcher (Robson-Kanu 65), Livermore, Morrison, Phillips (McClean 63), Chadli, Rondon. SUBS: Myhill, M Wilson, Yacob, Field.