Dean Kiely produced a fine second-half penalty save before Zoltan Gera netted a late winner to help West Brom leapfrog midlands rivals Wolves in the Coca-Cola Championship.
Kiely, signed from Portsmouth in January, kept out Paul Furlong's 66th-minute spot-kick with the scores level after substitute Dexter Blackstock had cancelled out Kevin Phillips' opener.
Gera then struck six minutes from time with his seventh goal of the season as Albion secured a priceless victory in the scramble for promotion.
The Hoops, however, now find themselves in deep relegation trouble just one point and one place above the bottom three.
Manager John Gregory resisted the urge to hand on-loan Wolves midfielder Rohan Ricketts his full debut, while defensive duo Neil Clement and Paul Robinson shrugged off heel and groin injury problems respectively to start for Albion.
The hosts went into the game buoyed by a three-game unbeaten streak that had seen them run out convincing 3-1 victors at Leicester last time out and they started brightly, Lee Cook firing wide from 12 yards inside 90 seconds.
Diomansy Kamara flashed wide from distance moments later in a blistering opening before midfielder Robert Koren curled just over from 25 yards.
Wales international Jason Koumas has been the cornerstone of Albion's push to rejoin the Barclays Premiership after reviving his career under Tony Mowbray.
He crashed a 25-yard free-kick straight at on-loan Derby goalkeeper Lee Camp as Albion continued to dominate proceedings while the home side were dealt a blow when playmaker Inigo Idiakez limped off injured and was replaced by Steve Lomas.
Kamara continued to cause problems with his pace and his cross-shot brought a fine sprawling save from Camp shortly before the half hour, with Phillips also testing the goalkeeper with a dipping 20-yard volley.
Kiely comfortably collected Adam Bolder's tame 25-yard effort shortly afterwards as a lively opening 45 minutes came to a close.
Mowbray opted to introduce Nathan Ellington at the interval in the hope of mustering an opening and it worked as Phillips broke the deadlock three minutes after the restart with a clinical finish.
Koumas played in Phillips after Albion broke quickly and the former England international beat two defenders before coolly placing the ball high past Camp from 12 yards.
Only a fine parry from Camp denied Phillips a second from 20 yards two minutes later before the England Under-21 stopper thwarted Ellington from a similar range as Albion threatened to run riot.
On the hour Gregory threw on youngster Blackstock and it proved an inspired decision as the former Southampton striker restored parity with virtually his first touch of the game.
Gareth Ainsworth made inroads down the right wing and whipped in a teasing cross to allow Blackstock to ghost in unmarked at the back post and head home convincingly from six yards.
The home side were then gifted the perfect opportunity to take the lead when Kiely clumsily upended Furlong in the area and referee Paul Armstrong had no choice but to award a penalty.
Furlong dusted himself to take the spot-ick himself but his low effort was dramatically tipped round the post by Kiely.
Veteran striker Furlong was then made to pay for his miss as Albion snatched the winner six minutes from time as Ellington produced an inch-perfect left-wing cross and Gera steered a six-yard volley past Camp to clinch all three points.
Teams:
QPR Camp, Mancienne, Stewart, Cullip, Bignot,Ainsworth (Ricketts 83), Idiakez (Lomas 14), Bolder, Cook,Furlong, Nygaard (Blackstock 61).
Subs Not Used: Cole, Kanyuka.
Booked: Blackstock, Ainsworth, Camp.
Goals: Blackstock 63.
West Brom Kiely, McShane, Clement, Sodje, Robinson, Koren,Greening, Carter (Ellington 46), Koumas, Kamara (Chaplow 85),Phillips (Gera 77).
Subs Not Used: Daniels, Albrechtsen.
Booked: Gera.
Goals: Phillips 48, Gera 84.
Att: 14,784
Ref: P Armstrong (Berkshire).