Saido Berahino’s low strike earned West Brom their first win over Manchester United at Old Trafford since 1978.
Morgan Amalfitano’s composed chip over David De Gea put West Brom in front after the break before Wayne Rooney’s free-kick brought United level.
Berahino then got his first Premier League goal, with Marouane Fellaini’s late effort ruled out for offside.
The defeat for United ensured their worst start to a league campaign after six games since 1989-90.
Less than a week on from their 4-1 hammering at the hands of Manchester City, United were desperate for three points and West Brom appeared to offer the perfect fixture, with the Baggies without a win over United since 1984 and none at Old Trafford since December 1978.
But they were to be outdone by Steve Clarke’s side, who ensured United remain on seven points after six games, the same tally they had collected 24 years ago in the old Division One.
The hosts created a few early chances despite a laboured start, with West Brom goalkeeper Boaz Myhill making a comfortable save from a Rooney header.
The Baggies should have taken the lead on the half-hour mark when Stephane Sessegnon headed wide after being found by midfielder Amalfitano, who is on loan from Marseille.
United boss David Moyes will have been frustrated with his side’s hesitant approach in the final third and, when they did breach the West Brom defence, the hosts were wasteful as Rooney headed over from Nani’s cross.
Berahino, who replaced the injured Scott Sinclair early in the first half, blasted over just before the break from close range with a number of red shirts on the line as Clarke’s side matched their more favoured hosts.
The Baggies continued to push forward after the break and Sessegnon slid a cute ball to Berahino, but the England Under-21 international’s low shot was saved by De Gea with his feet.
However, the opener came through the magic of Amalfitano, who ran through the United defence before he sent De Gea to ground with a clever faked shot then lifted the ball stylishly over the Spaniard.
United were still struggling to create chances but got some fortune when Rooney’s free-kick found its way to the net after it was missed by all the red shirts in the area and caught Myhill out.
The goals sparked the game into life, Jonas Olsson hitting the bar when he headed into the ground and the ball spun over De Gea, who then pulled off a superb finger-tip save to stop Amalfitano’s shot finding the top corner.
However, De Gea could do nothing to stop Berahino from putting West Brom back into the lead as the striker’s low strike nestled into the bottom corner soon after.
United pushed for an equaliser in the final stages but Fellaini was adjudged to be offside from Nani’s cross before fellow substitute Adnan Januzaj blasted wide.
Meanwhile, Manchester City missed the chance to go top of the Premier League following a shock defeat at Aston Villa.
Yaya Toure volleyed in the opener to give City hope of recording a first away win of the season.
Karim El Ahmadi’s close-range finish levelled matters only for Edin Dzeko to head City back into the lead.
But Villa equalised through Leandro Bacuna’s free-kick before snatching the three points when Andreas Weimann raced through and poked home late on.
Until those two late goals, City – bidding for a second Premier League title in three seasons – were on course to go top of the table, albeit for a few hours with Arsenal beating Swansea 2-1 in the late kick off to instead remain in first place.
The win for Villa, meanwhile, was their first at home this season and the second half performance will have given home fans plenty to be happy about – particularly the display of Dutch midfielder Bacuna.
The summer signing from FC Groningen provided the assist for Villa’s first, while his equalising free-kick – his first Premier League goal – would have been difficult for any goalkeeper to keep out.
City had arrived at Villa Park looking to maintain the momentum built up from their impressive 4-1 defeat of Manchester United last weekend, a third successive home win in the Premier League.
But for all their dominance at the Etihad Stadium this season, they have been disappointing on the road.
Manuel Pellegrini’s side had taken just one point from a possible six prior to travelling to Villa but a first away win looked promising for the visitors as they dominated early on.
The home side’s 3-5-2 formation was leaving them defensively fragile on the flanks and City repeatedly found spaces, with James Milner shooting wide while Samir Nasri’s deflected shot flew just wide of the upright.
City keeper Joe Hart had little to do in a first half Villa rarely managed to threaten in. The closest they came to scoring was a 25-yard free kick by El Ahmadi which sailed over the bar.
Ciaran Clark kept Villa in the game with a goal-line clearance from Toure’s effort as half time approached, but the midfielder was not to be denied his sixth of the season soon after.
A corner was whipped in by Nasri and though the ball was headed away from goal, it went only as far as Toure who hit a first-time volley underneath the dive of Guzan.
It was a lead the visitors fully deserved, but Villa were level just five minutes into the second half. Bacuna, 22, slipped a smart ball to send El Ahmadi clear and though the Morocco international appeared offside, the flag stayed down as he fired past Hart.
Villa’s delight was shortlived as City regained the lead minutes later, Dzeko powering a header past Guzan from Nasri’s corner.
Facing the prospect of a third successive home defeat, and having twice gone behind, Villa could easily have given into City but instead they grew into the game.
Captain Ron Vlaar volleyed into the hands of Hart from Vincent Kompany’s poor clearance before the hosts were presented an even better chance to equalise when they were awarded a 73rd minute free-kick 25 yards out. They took full advantage as Bacuna confidently stepped up to curl a sublime effort that gave Hart no chance.
Three minutes later, and with City still regrouping from that equaliser, Villa were gifted a decisive third goal.
Kompany and Matija Nastasic allowed Guzan’s delivery to bounce and Weimann took full advantage, racing towards goal before poking the ball beyond the onrushing Hart and into the net to seal a dramatic victory which lifts Paul Lambert’s side up to ninth in the Premier League table.