While third-placed Hull City could still technically catch Albion if they win their final game of the season at Ipswich Town and Albion lose at Queens Park Rangers, a superior goal difference of 12 effectively means Tony Mowbray's side are promoted.
But Albion did it the hard way against Southampton, coming from behind after substitute Adam Lallana had given the Saints a shock 77th-minute lead with his first goal for the club only three minutes after being sent on.
The former youth academy ace raced on to a pass from Jermaine Wright and showed a clean pair of heels to the Albion defence before scoring with a low drive.
The strike looked set to lift Southampton out of the relegation zone and send Leicester City crashing into the bottom three.
Northern Ireland international winger Brunt ended those hopes six minutes from time when he latched on to a long cross from Zoltan Gera and drilled an 18-yard shot through goalkeeper Richard Wright.
Albion should really have raced to a convincing victory as they laid siege to the Saints' goal in the second half.
But Ishmael Miller was denied by the impressive Wright, James Morrison fired wide and then 24-goal Kevin Phillips missed a golden chance when he headed a cross from Brunt over the bar.
Phillips was then denied by Wright before Lallana looked set to make Albion pay for their profligacy in front of goal.
Fortunately Brunt came to Albion's rescue with their 107th goal of the season to end their two-year absence from the top-flight.
But Southampton now have a massive fight on their hands to hand on to their Championship status. The point leaves them in the relegation zone and having to beat Sheffield United on Sunday and hope that other results go their way.
After a nervy opening to the match when Jhon Viafara and Inigo Idiakez both tried their luck from distance for Southampton, Albion took charge of affairs.
But their former centre-half, Chris Perry, proved to be a rock at the heart of the Southampton defence. Twice he made vital interventions to block goalbound shots from Phillips and Miller.
Albion skipper Jonathan Greening sent a long-range drive whistling over the bar before Marek Saganowski wasted Southampton's best chance of the half in the 37th minute.
The Poland striker was unable to get his shot on target from eight yards after the Albion defence got themselves in a terrible muddle.
It could have been a costly miss as Southampton then needed a smart save from Wright five minutes before half-time to snuff out a cross from the lively Robert Koren.