West Brom surrended the advantage in the battle for automatic promotion after falling to a 2-1 defeat at the John Smith's Stadium.
In a huge game at both ends of the table, the result saw Leeds United promoted to the Premier League and Huddersfield Town almost guarantee their safety, climbing six points clear of the relegation zone.
Winger Chris Willock put Huddersfield ahead from six yards out after a dangerous free-kick caused chaos in the box, before another free-kick saw right-back Dara O'Shea nod in at the far post to equalise.
West Brom pushed for the winner in the second half but were left stunned as substitute Emile Smith Rowe slotted in a late winner after a smart through ball from Lewis O'Brien.
Story of the match
Huddersfield were fast out the blocks and stormed into the lead ater just three minutes courtesy of Chris Willock.
The on-loan Benfica winger drove home a loose ball from 6 yards after a dangerous Juninho Bacuna free kick rebounded off the unsighted goalkeeper Sam Johnstone.
Willock was the bright spark in the early stages as West Brom looked stunned by Huddersfield's strong start, but Lewis O'Brien was unable to capitalise further, slicing his effort after more good work down the right from Willock.
West Brom needed a wake up call and it was Romaine Sawyers who was caught napping next as he dallied on the ball outside his own box. The ball broke to experienced striker Fraizer Campbell but he fired wildly over the bar.
The pressure was building for the Baggies as they needed to win to keep third placed Brentford at bay, and Huddersield frustrated their opposition with a number of fouls.
West Brom grew into the game and as Huddersfield retreated into their own half, the chances started to come.
Hal Robson-Kanu clipped a ball into the back post from the edge of the area and O'Shea arrived at the back post just in time to cushion a header back into the danger zone. The ball dropped across the 6 yard box but West Brom were unable to apply the finishing touch amidst a risky challenge from Christopher Schindler.
But it didn't take long before West Brom did find the equaliser. Another free kick from Matheus Pereira caught the goalkeeper off-guard and he could only palm the ball into the path of the waiting O'Shea at the back post, who nodded home to level in the 42nd minute despite calls for offside.
The Baggies smelt blood after the leveller but Robson-Kanu was unable to get on the end of a Grady Diangana pull back following some neat build up play, and Semi Ajayi had an effort blocked from a corner.
Huddersfield had the first chance of the second half - again from a set peice - but neither Schindler or Campbell could get on the end of an inviting ball from Bacuna.
Slavan Billic brought on Kamil Grosicki and Filip Krovinovic at half time but it was Pereira who continued to shine, having one shot blocked, one parried away by Jonas Lossl and another over the bar from distance.
West Brom continued to probe but Callum Robinson was unable to take advantage of their best opportunity, shooting weakly into the hands of Lossl after some intricate play around the box.
And it was Huddersfield that dealt the killer blow in the 86th minute. Lewis O'Brien picked the ball up in the centre of the pitch and slotted a deft through ball into the path of substitue Emile Smith Rowe, who dispatched the ball into the far corner from just inside the box.
The Baggies piled on the pressure as they desperately looked to get something from the game, but Huddersfield held on - sending their Yorkshire rivals Leeds into the Premier League and putting Brentford in charge in the race for 2nd place.
Play-offs for West Brom?
Huddersfield's much improved post-lockdown defence laid the foundations for the win as they frustrated West Brom for large parts of the game.
West Brom did create enough to get something from the game, with playmaker Matheus Pereira looking bright throughout.
But there was no doubt that Slavan Billic's side were far from their best and their manager will have a job on his hands to pick his team up with the loss leaving the Baggies facing the lottery of the play-offs.
Man of the Match: Matheus Pereira
Such a devasting result for West Brom won't be enjoyable for any of the players, but Matheus Pereira can leave the pitch with his head held high.
The talisman was his creative best throughout, chipping in with an assist, making things happen in attack and drawing a number of fouls from the opposition.