Stephen Elliott stepped off the bench to fire Sunderland to the Coca-Cola Championship title with a brilliant late winner that could wreck West Ham's play-off hopes.
The pacy striker, on as a substitute for Marcus Stewart, struck with three minutes left as the Black Cats fought back from a goal down at half-time to ensure Mick McCarthy's men clinched the crown.
Marlon Harewood put the home side ahead just before the break, but Sunderland were much better after half-time and deserved Julio Arca's 52nd-minute equaliser.
West Ham may now need to win at Watford next Sunday to have a chance of a play-off spot.
West Ham made the better start, but Sunderland thought they had taken the lead in the 11th minute.
Dean Whitehead's close-range effort from Arca's left-wing cross was saved by James Walker and Chris Brown prodded home the rebound - only to see his effort was ruled out for offside.
Buoyed by their good fortune, West Ham took control, but strikers Bobby Zamora - in for the injured Teddy Sheringham - and Harewood struggled to turn Sunderland centre-backs Gary Breen - a former West Ham player - and Steve Caldwell early on.
Alan Pardew's men almost scored in the 19th minute, Ben Alnwick making a superb save from Harewood's ferocious 20-yard strike following a clever free-kick involving Harewood, Shaun Newton and Matthew Etherington.
Zamora thought he had given West Ham the lead two minutes later, but Alnwick had already caught Etherington's cross before Zamora's head made contact with the ball and the strike was disallowed.
Harewood then stabbed wide from 10 yards after an alert one-two with Zamora after 35 minutes.
The breakthrough came two minutes before half-time, Harewood collecting Etherington's pass and drilling a left-foot shot in off the near post from close range.
Sunderland were far more imposing after the break, and equalised in the 52nd minute.
Attempting to dispossess Liam Lawrence, defender Elliott Ward could only turn the ball past 'keeper Walker and Arca was quickest to react, forcing the ball home under pressure from Anton Ferdinand from virtually on the goal-line.
West Ham were creating few chances but 11 minutes from time, Steve Caldwell did well to block Nigel Reo-Coker's goalbound effort.
Three minutes later, Hayden Mullins sent a volley just wide after West Ham substitute Sergei Rebrov's free-kick had been blocked.
At the other end, Walker saved superbly to prevent Elliott winning it for Sunderland.
But Elliott was not to be denied and, after out-pacing Ward, he curled a left-footer into the bottom right corner from 18 yards.
Teams
West Ham Walker, Powell, Ferdinand, Ward, Repka,
Etherington (Noble 73), Reo-Coker, Mullins, Newton (Rebrov 81),
Zamora, Harewood.
Subs Not Used: Bywater, Fletcher, Chadwick.
Booked: Repka, Harewood, Noble.
Goals: Harewood 43.
Sunderland Alnwick, McCartney, Breen (Danny Collins 74),
Caldwell, Wright, Lawrence, Whitehead, Robinson, Arca,
Brown (Deane 89), Stewart (Elliott 75).
Subs Not Used: Welsh, Carson.
Booked: Alnwick.
Goals: Arca 52, Elliott 87.
Att: 33,482
Ref: R Beeby (Northamptonshire).
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