Carlos Corberan’s side crashed to three second-half goals, the all-important opener netted by midfielder Will Smallbone after a poor Grady Diangana error inside five minutes of the second period.
The visitors were unable to recover and top goalscorer Adam Armstrong delivered with his 22nd and 23rd goals of the campaign inside the final 12 minutes to kill off any lingering hopes from the 2,000-strong travelling Baggies contingent.
Cedric Kipre nodded in with the final touch of the ball deep into stoppage time but the consolation was barely noticed.
The Baggies gave themselves every chance heading to Hampshire for the second leg after a stalemate at The Hawthorns and the visitors were relatively content at a goalless half-time at St Mary’s.
Southampton had been the brighter side but Coberan’s pragmatic approach held its own. The game was arguably lost in the first five minutes of the second period, though, and while Albion showed small shoots of a response, Southampton’s firepower, quality and left-over Premier League resources shone through.
It took until either side of the hour, at a goal down, for the visitors to test Alex McCarthy at all. The 12-point gulf in quality between the fourth and fifth-placed sides in the Championship shone through when push came to shove in leg two. Corberan’s weary, battle-hardened troops had fought so hard as best of the rest in the division, but it will end with third against fourth as Saints tackle Leeds at Wembley.
Southampton fans stormed the pitch at full-time, which led to some unnecessary goading in front of the away end on a painful climax to the campaign.
Corberan’s sole change to the starting XI from the first leg was at the top of his attack as captain Jed Wallace came in for Brandon Thomas-Asante, who dropped to the bench, to lead the line.
The other pre-match note of interest was among the substitutes, where Josh Maja was once again omitted from the 20-man matchday squad having been decided not ready for the contest by boss Corberan. The head coach said last week the striker, injured for much of the campaign, was not physically ready for the calibre of fixture.
There was not just a big-match feeling around St Mary’s ahead of the second leg, but also an air of confidence that Southampton’s place at Wembley to face Leeds was all-but booked.
The home side, predictably, looked to gain control of the contest from the outset following a fire show as the players entered the pitch at St Mary’s.
Russell Martin, who had brought Ryan Fraser and David Brooks into the side, saw right-back Kyle Walker-Peters have an effort from the left well blocked and Brooks – a scorer at The Hawthorns in February – combined dangerously with Smallbone down the right.
Albion keeper Alex Palmer wasn’t afraid to play with some leftover pre-match fire with some close pieces of control to escape Armstrong and Brooks inside his own box. The latter nearly caught the shot-stopper.
The Saints had most of the ball but Albion forged half-a-sight after 10 minutes. Attacking towards where the boisterous away fans were housed, Alex Mowatt and Mikey Johnston exchanged a short corner and the former looped a cross to the back post towards Diangana.
The Baggies attacker found some space but could only nod wide of Alex McCarthy’s right post.
Referee Tim Robinson made the right call to wave away penalty appeals on Brooks under pressure from Conor Townsend. Contact was very minimal and soft.
Bournemouth loanee Brooks spurned the biggest chance of the first period narrowly wide of the near post as he connected to a low Armstrong cross from the right. Southampton’s threat, though, came mostly down the left through Newcastle loanee Fraser, who was up against Tom Fellows in both directions and on occasion managed to get on the blindside of the Albion youngsters, but the touch let the home player down.
Fellows can close in the other direction as his hung-up cross from the right byline almost crept in at the far post but for McCarthy’s crucial intervention in goal.
The Baggies had another spell before the half hour as Kyle Bartley and then Diangana attacked a couple of successive Alex Mowatt deliveries but couldn’t find a clear route to goal.
Southampton stepped up during the final 15 minutes of the first period. Bartley had to stand tall with some excellent pieces of defending after dallying from Mowatt and lapses from Diangana’s first touch. Albion’s attack had lacked somewhat with Mikey Johnston not involved enough and all of ex-Portsmouth man Wallace’s hard work coming without the ball.
Palmer grasped at a low drive from Brooks from range and kept hold of the ball to prevent a rebound.
The half-time whistle marked three periods of goalless action between the two sides.
Any Albion plans for a route to Wembley had to be reconfigured less than five minutes into the second half.
Diangana had struggled all evening and his ball across midfield was loose and easily cut out by goalscorer Smallbone.
Smallbone worked it to dangerman Brooks, who turned a good pass back to the midfielder, who had worked his way towards the box. Smallbone, slightly right of the box, let fly and his arrowed strike kissed Palmer’s near post on its way in. A needless Baggies hammerblow.
Albion had to respond and tried to, roared on by the travelling support, who weren’t ready to give up.
A mighty chance came minutes after the opener. Diangana and Fellows combined on the right and the latter charged clear. His low cross was perfectly pulled back for Mowatt to strike first time, but his effort struck Taylor Harwood-Bellis. The strike looked to be heading in.
At the other end Saints pleaded for a penalty on the hour as last-man Kipre tried to deny Brooks after Palmer’s hurried clearance. Kipre won the ball but went through his man to get it and replays appeared ominous. Referee Robinson was unmoved.
Albion tried to hit back and tested McCarthy through a Mowatt corner as Darnell Furlong struck a loose ball well. His rising effort was acrobatically tipped over by the veteran keeper.
Corberan’s triple change called on Thomas-Asante, John Swift and Yann M’Vila and Swift’s free-kick delivery found Kipre who volleyed into the side netting from an offside position after Armstrong’s finish had also been ruled out for offside.
The Baggies tried to manoeuvre a way forward but it was the hosts that dealt the killer blow 12 minutes from time.
Albion were slow to close down home top scorer Armstrong on the left of the box and the frontman’s driven left-footed was low and true into the bottom corner across Palmer, who couldn’t get down. Fellows nor Furlong could close down the attacker.
Albion sub Adam Reach sent a difficult volley wide before the home side were awarded a penalty for Fellows’ foul on Ryan Manning in front of despondent Baggies.
Armstrong netted the penalty with conviction, something he and Southampton have shown for much of the season and Albion have, very often, lacked. Even Kipre’s nodded consolation was a barely-noticed footnote and far little, too late.
Southampton (4-3-3): McCarthy; Walker-Peters, Bednarek, Harwood-Bellis, Stephens (c); Downes, Smallbone, Aribo; Brooks (Edozie, 74), A Armstrong; Fraser (Manning, 66).
Subs not used: Lumley, Stewart, Bree, Mara, Rothwell, Sulemana, Charles.
Albion (4-2-3-1): Palmer; Furlong, Bartley, Kipre, Townsend (Reach, 72); Yokuslu (M’Vila, 65), Mowatt (Weimann, 81); Fellows, Diangana (Swift, 65), Johnston; Wallace (c) (Thomas-Asante, 65).
Subs not used: Griffiths, Ajayi, Chalobah, Phillips.
Attendance: 30,712
Referee: Tim Robinson