West Brom moved a step closer to relegation at St Mary's where James Beattie's 19th goal of the season settled a fiercely contested match.
Beattie struck in the eighth minute with a long distance right foot shot after Paul Telfer and Brett Ormerod had combined.
Gary Megson's team had plenty of possession but Southampton goalkeeper Antti Niemi did not have a single shot to save.
The nearest Albion came to rescuing a point was in the 88th minute when Lee Hughes mis-hit a shot from eight yards and in the chaos that followed Daniele Dichio hit the outside of the post when it looked easier to score.
Albion were indebted to keeper Russell Hoult who made an outstanding first-half save from Ormerod and then turned away at full stretch a low shot from substitute David Prutton in the 83rd minute.
The Baggies did not force their first corner until the 62nd minute but could have been level earlier in the match when Jason Koumas bent a free-kick narrowly wide with Niemi beaten.
Referee Dermot Gallagher was a busy man halting play for more than forty fouls and issuing four yellow cards.
Megson was furious when Gallagher turned down appeals for handball in the Southampton area but Albion had little to show for their aggression, determination and possession.
Ronnie Wallwork and Sean Gregan should have done better with shooting chances inside the area as Southampton lived dangerously.
But the win ended a run of three successive Premiership defeats and was the perfect morale booster for the FA Cup quarter-final clash with Wolves at St Mary's next Sunday.