ALBION suffered their first defeat in five Barclays Premier League games as new head coach Pepe Mel watched from the stands ready to take the reins.
Adam Lallana fired the only goal of the game in the 66th minute in a closely-fought St Mary’s Stadium encounter.
Both goalkeepers were rarely tested but Ben Foster was the slightly busier of the two as the Baggies lacked the cutting edge to open up the hosts’ defence.
But the Baggies were left ruing missing a golden opportunity to steal a point in the final minute when Shane Long was denied by Artur Boruc's wonder save from close range.
Keith Downing made three changes to the side that beat Newcastle on New Year’s Day with Steven Reid, James Morrison and Long earning recalls to the starting XI.
Mauricio Pochettino also made three changes from his last league line-up, with Boruc, Nathaniel Clyne and Rickie Lambert drafted in.
The first effort on goal came from the hosts in the eighth minute as Adam Lallana spun on the edge of the box and fired in a shot that Foster held comfortably.
Moments later, Lallana picked out Rickie Lambert in the penalty area and the England striker scuffed a left-foot shot across goal from 15 yards.
Gareth McAuley did brilliantly in the 22nd minute to beat Jay Rodriguez to Luke Shaw’s whipped-in cross at the near post - and Foster gathered the loose ball.
Nicolas Anelka was brought down by Morgan Schneiderlin in a central position on 26 and Reid took the opportunity to blaze the 30-yard free-kick just over the bar.
Albion broke quickly in the 43rd minute as Youssouf Mulumbu set Long off on a run down the right. Morrison burst clear down the middle but the Ireland international’s pass lacked pace and the chance was gone as the Saints defence recovered.
Seconds later, at the other end, Steven Davis dinked the ball to the back post and Lambert smashed a volley two yards over from 15 yards.
The Baggies started the second half positively and, within three minutes, Long got on the end of Liam Ridgewell’s left-wing cross but could not get enough purchase on his header to beat Boruc.
Three minutes later, Mulumbu lost possession to Rodriguez in the middle of the park and the ex-Burnley man had McAuley twisting and turning before firing wide of the near post from the edge of the box.
Seconds later, Reid made an important block on Davis’ goalbound drive from just inside the area.
Approaching the hour-mark, Long let slip the opportunity to set up Morgan Amalfitano in the six-yard box.
Back came Saints and, a minute later, McAuley was called upon to block Lambert’s fierce 20-yard drive.
The hosts finally found a way through on 66 as Lallana latched onto sub Gaston Ramirez’s deft chip and coolly slotted past the onrushing Foster to open the scoring.
Downing, who switched to three centre-halves midway through the second half, made his first change in personnel on 77 as Saido Berahino came on for Anelka.
A second change swiftly followed on 80 as McAuley, who looked to be clutching his hamstring, made way for Zoltan Gera, with Reid dropping to centre-half.
Things opened up for Morrison on the edge of the area with two minutes to go but he sliced his shot horribly wide.
However, the Baggies should have levelled in the final minute as final sub Vydra slid Ridgewell in behind and he centred for Long, who was denied from six yards by the sprawling Boruc.
SOUTHAMPTON (4-1-2-3): Boruc; Clyne, Fonte, Lovren, Shaw; Cork (Ramirez 62); Davis, Scheneiderlin; Lallana (Ward-Prowse 84), Lambert, Rodriguez (Do Prado 90+1). Subs not used: K Davis (gk), Yoshida, Hooiveld, Gallagher.
ALBION (4-4-1-1): Foster; Reid, McAuley (Gera 80), Olsson, Ridgewell; Amalfitano (Vydra 86), Morrison, Mulumbu, Brunt; Anelka (Berahino 77); Long. Subs not used: Myhill (gk), Lugano, Sinclair, Yacob.
GOALS: SOUTHAMPTON – Lallana (66).
BOOKINGS: SOUTHAMPTON – Lovren (foul 27). ALBION – Long (foul 58), Olsson (foul 73).
REFEREE: H Webb (S Yorkshire).
ATTENDANCE: 28,610 (Albion fans 1,621).
ALBION STARMAN: Chris Brunt.