Sensational strikes from Carlos Edwards and Grant Leadbitter ensured Sunderland rose to the summit of the Coca-Cola Championship for the first time this season.
Marek Saganowski's sixth goal in four games put the Saints ahead after 67 minutes but the Black Cats showed the resilience that has seen them go on a 15-match unbeaten run as two goals in the final 13 minutes secured a memorable three points.
Edwards has been a revelation following his January transfer from Luton and equalised with a sensational drive from 25 yards that clipped the underside of the crossbar on its way in.
And just as the game looked to be petering out to a draw, substitute Leadbitter crashed home from 25 yards to send the travelling support into raptures.
Defeat for third-placed Birmingham and a draw for Coventry at leaders Derby earlier in the day means Roy Keane's side are now in pole position for a return to the Barclays Premiership at the first attempt.
Keane has yet to name the same starting XI in consecutive matches since he took over in August and it was no different at St Mary's.
There was a first start since January for Republic of Ireland striker Steven Elliott, who partnered David Connolly in attack, while Tobias Hysen and Dwight Yorke were preferred in midfield to Dean Whitehead and Ross Wallace.
George Burley also rang the changes following the win over Luton Town on Saturday with Chris Baird, Alexander Ostlund, Djamel Belmadi and Nathan Dyer all earning recalls.
Both sides started tentatively and it was not until the 14th minute that the first clear opportunity was created.
Liam Miller did brilliantly to dummy Edwards' neat cross and Connolly was perfectly positioned to swivel and strike a crisp drive, which clipped the outside of Bartosz Bialkowski's left-hand post.
That seemed to galvanise the visitors and they threatened twice more through Elliott and Yorke.
Firstly, Elliott saw his angled drive parried into the side netting by Bialkowski after he had been released by Tobias Hysen's accurate pass.
And from the resulting corner former Manchester United man Yorke could only head over from ten yards when it looked easier to score.
Sunderland were threatening to run riot, but Southampton slowly edged their way back into the game and created a clear opening of their own after 20 minutes.
Leon Best, who is keeping top-scorer Grzegorz Rasiak out of the side, took advantage of Jonny Evans' hesitancy on the edge of the penalty area, but his low drive was well kept out by the outstretched leg of Darren Ward.
Sunderland wasted a golden opportunity to take the lead on the stroke of half-time as Edwards lashed wildly over the crossbar after being released by Elliott's pass.
Southampton started the second half brightly and sought-after teenager Gareth Bale signalled their intentions with a left-footed strike from 25 yards that fizzed past Ward's upright.
Chances were at a premium in the early stages of the second half and when one did finally fall to Sunderland, Yorke was only able to scuff his half-volley wide after Baird's headed clearance had found him 15 yards from goal.
But Southampton finally broke the deadlock after 67 minutes as Saganowski continued his hot streak.
Belmadi's deep ball was cushioned towards the six-yard box by substitute Danny Guthrie and the Poland international stole a march on Nyron Nosworthy to steer a volley past the flailing Ward.
And they came agonisingly close to doubling their advantage just minutes later as John Viafara's drive from an acute angle was once again kept out by the feet of Ward.
After the game Sunderland manager Roy Keane said: 'Now is not the time to reflect on what has happened. We cannot do that with four games still to go.
'We have only been top for five minutes so it is difficult to say how it feels.
'We have a tough game with Queens Park Rangers at the weekend and that is the next challenge.'
