Juan Mata's early red card was the catalyst for Manchester United's downfall as Salomon Rondon's goal saw West Brom halt a run of four successive wins for Louis van Gaal's side.
Talk of a top-four challenge has followed the recent upturn in fortunes at Old Trafford, but Albion ensured that a key week for visitors United would start with a loss.
Mata's foolish sending-off for two bookings midway through the first half gave West Brom an advantage they capitalised on, with Rondon's fine finish earning a 1-0 victory that almost certainly secures survival for Tony Pulis's side.
The Venezuela striker's fourth goal in six Premier League matches condemned United to a first league defeat at The Hawthorns since March 1984 and saw them fall behind in the race for the Champions League.
West Brom were perhaps unfortunate not to take the lead in a first half that saw Mata pick up the first red card of his club career -- United's first sending-off of the season -- after collecting two bookings in less than three minutes.
Van Gaal's men showed signs of recovery as they returned from half-time brightly, only for Rondon to lash home and end Albion's run of 11 winless league fixtures at home to the Reds.
Defeat leaves United three points behind neighbours Manchester City, who also have a game in hand, and the loss had appeared a distinct possibility after a lacklustre opening period.
Anthony Martial's tame effort after two minutes was all Van Gaal's side could muster, whereas the hosts proved a threat from the outset.
Rondon's directness made him a nuisance, but it was his vision that almost undid United when his clipped ball to Craig Dawson was headed wide in the ninth minute.
Saido Berahino's scuffed attempt from a clever Craig Gardner header was blocked by the returning Chris Smalling as the home side continued to press, with the home fans unhappy to see Daley Blind's high foot on Berahino adjudged to have been outside the box.
The noise inside the Hawthorns rose several notches after that decision, and shortly after Gareth McAuley had headed inches wide, unaware the offside flag was raised, it went up further.
Less than three minutes after being booked for encroaching on a quickly-taken free kick, Mata's challenge on former teammate Darren Fletcher brought him a second yellow.
That naturally invited pressure on United's backline, but the closest Albion came before half-time was an audacious half-volley from Dawson that dropped wide.
Van Gaal's men attempted to capitalise on those missed Baggies opportunities as they started the second half with some sharp football.
Marcus Rashford was largely anonymous, but saw a snapshot deflect wide after exceptional work down the left by Martial.
Jesse Lingard was the next to threaten, cutting back and curling a strike just over the bar from the edge of the box.
Ander Herrera hit a free kick into the wall as United continued to threaten a West Brom defence that had been destabilised by McAuley limping off -- only for his replacement Sebastien Pocognoli to create the crucial goal.
After referee Mike Dean had waved play on, the ball was played out wide for the Belgian full-back to fizz in a cross that Rondon controlled well before firing home sweetly.
It was a 66th-minute setback that United rarely threatened to recover from, with the introduction of Memphis Depay in place of 18-year-old Rashford failing to kick-start the visitors.
Most of the play was in their half but, like the visitors, Albion were unable to create any clear-cut chances -- not that it mattered to them in the end.