SUPER-SUB Chris Brunt's 84th-minute leveler all-but booked Albion's place in the Premiership next season - and fired them back to the Championship summit.
Third-placed Hull would need to win at Ipswich on the final day of the season on Sunday, overcome a staggering 12-goal deficit in the goal difference stakes, and see the Baggies lose at QPR, to deny Tony Mowbray's men a place back in the top flight.
After a tame first-half, Albion spent most of the second period on the front foot but were constantly denied by a resolute Southampton defence and keeper Richard Wright's heroics.
The Baggies' automatic promotion bid looked to be going right down to the wire when Saints sub Adam Lallana opened the scoring against the run of play 12 minutes from time.
But Brunt effectively ended Albion's two-year exile from the Premiership by smashing home his fourth of the season six minutes later.
The Hawthorns literally shook when Brunt's piledriver crept through the otherwise impeccable Wright's legs and a win at Loftus Road will earn the Baggies their first title in 88 years.
Albion started the game knowing a win would guarantee them a top flight return and that a point should be enough to effectively clinch promotion due to their superior goal difference.
Southampton's plight could not have been more contrasting, with the side managed by ex-Baggies No.2 Nigel Pearson needing a win to lift them out of the bottom three.
Mowbray rang one change for the club's final home league game of the season, replacing Roman Bednar with Ishmael Miller in attack.
An expectant, sell-out, home crowd got The Hawthorns rocking - but it was Southampton who recorded the first shot on goal in the seventh minute, with Jhon Viafara shooting well wide from 25 yards.
Fellow midfielder Inigo Idiakez was just as wayward from similar distance seconds later.
Albion had taken a while to settle into their stride, but it took a vital last ditch challenge by former Albion defender Chris Perry in the 15th minute to deny Kevin Phillips a clear sight of goal.
Seconds later, James Morrison fired well wide from 20 yards.
Back at the other end, the steady stream of disappointing long-range efforts continued when Stern John dragged a 20-yard half-volley wide.
Back came Albion and Perry again proved a thorn in Albion's side when he dispossessed Miller just as the striker was about to pull the trigger from 15 yards.
Saints keeper Wright then had to be on his toes to race out of his area and clear to prevent Robert Koren getting on the end of Zoltan Gera's slick through-ball.
There was much more purpose about the Baggies' play now and on-loan West Ham keeper Wright showed a great pair of hands to clutch Gera's dangerous right-wing cross, with Phillips lurking.
But Albion had yet to create a clear chance as the game crept into the 34th minute, when Neil Clement failed to trouble Richard Wright with an ambitious 30-yarder.
However, moments later, Jonathan Greening went much closer with a 25-yard missile which fizzed just over the Southampton bar.
But the Baggies had a real let off in the 37th minute.
On a rare Saints attack, Jason Euell's left-wing cross bounced kindly off Clement for Marek Saganowski, who wastefully blazed over from ten yards.
The game had now come to life and, after a Koren cross had cannoned off three Saints defenders in the box, Richard Wright pounced on the ball in the nick of time to deny Miller and Phillips a tap-in.
A half which, although competitive, had largely been a non-event, came to an end after Gera had spun on Carl Hoefkens' neat lay-off and fired straight down Richard Wright's throat from 20 yards.
Gera was in the thick of it again at the start of the second half when he towered at the far post to head Greening's deep corner goalwards, but the ball flew straight at Richard Wright.
Miller had the home hordes on their feet moments later when he charged into the box and beat Youssef Safri and Perry before seeing his near-post shot blocked by Richard Wright.
Albion were now in full flow and a great move involving Paul Robinson and Phillips resulted in Morrison firing a half-chance wide from 18 yards.
Miller went much closer in the 57th minute when his left-footed shot from just inside the box deflected off Perry and screwed agonisingly wide after neat link-up play by Clement and Gera.
Mowbray made his first change in the 64th minute when Brunt replaced Morrison, who had suffered a knock moments earlier.
And his second followed five minutes later, when Bednar came on for Miller.
Albion were keeping the Saints pegged inside their own half but just could not find a breakthrough.
Brunt made his first contribution in the 71st minute when his left-wing centre was headed just wide by Phillips at the back stick.
There was further Albion agony in the 75th minute when Richard Wright pulled off a stunning point-blank save to keep out Phillips' header after Gera had nodded Clement's diagonal free-kick across the six-yard box.
Southampton finally threatened again moments later when sub Lallana raced into the box and fired straight at Kiely.
But the visitors stunned The Hawthorns in the 78th minute by snatching a shock lead.
That man Lallana raced clear of the Baggies defence to control Jermaine Wright's defence-splitting pass before clinically rifling the ball inside Kiely's far post.
Clement wasted a gilt-edged chance in the 81st minute when he directed a free six-yard header from Brunt's corner into the ground and over the bar.
Sub Luke Moore also went mightily close with a header from another Brunt flag-kick.
But The Hawthorns erupted in the 84th minute.
Viafara failed to cut out Bednar's centre and Brunt set himself up with a wonderful first touch before rifling the ball through Wright from just inside the box.
Albion pressed forward for a second goal which would put their Premiership status beyond any mathematical doubt and, three minutes from time, Moore volleyed Brunt's cross just wide.
But the game finished all square and that was enough for the home hordes who flooded the pitch at the final whistle to get the promotion party started.
ALBION (4-4-2): Kiely; Hoefkens, Barnett, Clement, Robinson; Gera, Koren, Greening (Moore 81), Morrison (Brunt 64); Phillips, Miller (Bednar 69). Subs not used: Danek (gk), Albrechtsen.
SOUTHAMPTON (4-4-2): R Wright; J Wright, Lucketti, Perry, Surman; Safri (Dyer 63), Viafara, Idiakez, Euell; Saganowski (Lallana 74), John (Wright-Phillips 74). Subs not used: Davis (gk), Licka.
GOALS: ALBION - Brunt (84). SOUTHAMPTON - Lallana (78).
BOOKINGS: ALBION - Hoefkens (foul 37), Clement (foul 79). SOUTHAMPTON - Euell (foul 42), Perry (foul 49), Idiakez (foul 74).
REFEREE: L Mason (Lancashire).
ATTENDANCE: 26,167.
ALBION STARMAN: Paul Robinson.
PIC: GET IN THERE...Brunt celebrates his all-important equaliser
