Three points looked to be going to Saints after Billy Sharp poked home late on, but the game changed in the final minute of added time when the Pompey midfielder lashed home.
Southampton originally took the lead midway through the first half through Sharp but Chris Maguire's rasping effort levelled for Portsmouth.
Jamie Ashdown's fantastic performance in the Pompey goal seemed to secure the visitors a point until Sharp put Southampton back ahead in the 89th minute.
However, Norris struck deep into stoppage time to break home fans' hearts - even though they return to the top of the npower Championship on goal difference.
St Mary's was rocking as the match began and the intensity translated onto the pitch, with referee Neil Swarbrick issuing the game's first yellow card after 69 seconds.
Morgan Schneiderlin was booked for hauling back David Norris and was soon forced off the field through injury after a seventh minute collision with Luke Varney.
Jack Cork came on in his place but not until after Maguire had picked up the second booking of a frantic opening.
Both sides were struggling to get a foothold, but Pompey came close to breaking the deadlock when Kelvin Davis was forced to parry a Kelvin Etuhu drive following a defensive mishap.
It was Saints, though, that were looking the more assured side and, after Danny Fox saw a long-range effort go close and Ricardo Rocha was booked, they broke the deadlock.
Pompey failed to clear a corner and Sharp collected a pass from Rickie Lambert in the box, before rifling home in front of the raucous Northam End.
The former Doncaster striker was booked for putting his finger to his lips in front of the home support after the 27th minute strike and card-happy referee Swarbrick soon issued his fifth yellow card of the half to Scott Allan.
Southampton were looking comfortable but a thunderbolt out of the blue drew Pompey level in the 36th minute.
Maguire found time on the edge of the box to hit a venomous shot that beat Davis and flew into the top corner.
The goal sparked the 3,034 visiting supporters into voice, but Saints dominated until the end of period.
Pompey began the second half brightly and should have gone ahead inside a minute of the restart, but Maguire inexplicably scuffed wide with the goal gaping.
Saints soon jolted back into life and the visitors had goalkeeper Ashdown to thank for keeping the scores level.
The Portsmouth goalkeeper first thwarted Adam Lallana's expertly-struck volley and then tipped over Jose Fonte's goalbound header from the resulting corner.
Lallana fizzed an effort wide after neat play with Sharp, before Portsmouth enjoyed a spell in the ascendancy at the other end.
As the game entered the final 10 minutes, Portsmouth were penned back in their own half.
Fonte came close and Lallana toppled over after rounding the goalkeeper, before Rocha almost put into his own net.
Southampton continued to press and got the goal they deserved in controversial fashion in the 89th minute.
Sharp poked home at the far post and saw his goal disallowed for offside, but the decision was reversed after a chat between the referee and linesman.
However, things changed again in stoppage time when Norris struck home to the delight of the visiting fans.
Leeds were more than happy to soak up the pressure, and their appetite to hit on the break almost paid dividends when McCormack's 20-yard effort rebounded to safety off an upright.
McCormack then teed up Snodgrass 12 yards out but Federici produced a good save, before the former Cardiff man's thunderous free-kick brought a fumble out of the Australian stopper.
At the other end, Pearce's goalbound header was cleared off the line while Le Fondre followed up McAnuff's deflected effort to slide home - only for his celebrations to be cut short by an offside flag.
But with seven minutes remaining, Reading finally found a way through.
Jimmy Kebe's deep cross to the back post was met by Hal Robson-Kanu who sent a delicate header back across goal for Le Fondre to chest home.
Two minutes later Le Fondre completed the job. Again the required finish came from less than two yards out, but the former Rotherham man was on hand to knock in Jason Roberts' cross-shot and spark mass delirium amongst the home supporters.
In the late kick-off, West Ham set a new club record of twelve away league wins and stayed within touching distance of the automatic promotion spots with a comfortable 4-0 win against Barnsley at Oakwell.
Kevin Nolan capitalised on an unfortunate ricochet to flick the ball past David Button from close range with only seven minutes on the clock following a corner.
The Tykes were the architects of their own downfall as the visitors were afforded far too much time on the ball throughout.
And it was that lack of pressure on the ball that allowed Nicky Maynard the time and space to pick his spot from outside the box following a neat passing move with a little over twenty minutes gone.
The Hammers, in contrast, put the home side under almost constant pressure.
And that intense pressure - this time applied by Maynard - forced Button into a loose clearance, which Mark Noble collected and from an acute angle chipped over a retreating Barnsley defence to give the visitors a three goal advantage at the interval.
Barnsley made more of a fist of the opening exchanges of the second period but if there were any doubts as to the destination of the points they were firmly extinguished when former Barnsley man Ricardo Vaz Te tapped in from close range after a good run and shot from George McCartney.
The scoreline would have had an extra gloss if Maynard had managed to add to his tally, but first the crossbar and then the assistant referee's flag were to deny the former Bristol City man.
It was to matter little however as the Hammers will go into Monday's crunch clash with Birmingham in buoyant mood after a very professional performance.
Elsewhere, Blackpool moved three points clear of sixth-placed Brighton in the race for the play-offs with a 2-0 victory against Watford at Vicarage Road thanks to goals from Stephen Dobbie either side of half-time.
The first was a strike of some quality, picking up the ball near the halfway line before slaloming past a number of flailing Watford challenges and then toe-poking past Tomasz Kuszczak in the home goal.
The second half was a more keenly fought affair until Kuszczak brought down Thomas Ince with 70 minutes on the clock as the youngster latched on to a Matt Phillips pass.
Dobbie made no mistake from twelve yards to secure all three points.
Brighton and Hove Albion, level on points with Blackpool coming into this round of fixtures, lost ground in the race for the play-offs following a disappointing 1-0 defeat against Burnley at Turf Moor.
Charlie Austin with the composed finish following a fortunate break of the ball in the 23rd minute.
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