Paul Furlong's missed spot-kick proved decisive, as Zoltan Gera broke Rangers hearts with a late, late winner.
The R's succumbed to a first defeat in four outings, when the Albion substitute powered home a stunning back post volley in the 84th minute.
Earlier, Kevin Phillips put Albion in front inside three second half minutes, only for R's substitute Dexter Blackstock to equalise with his first touch of the afternoon 14 minutes later.
Furlong was denied from the spot by Dean Kiely soon after, before Gera came up trumps for the visitors in the dying embers of a truly enthralling second period.
In truth, Rangers created the better opportunities in a free-flowing opening 45 minutes.
Gareth Ainsworth was at the heart of the R's positive display, providing chances for both Furlong and Marc Nygaard.
Furlong's effort from eight-yards on the quarter hour finished inches wide of the post, while Nygaard headed over from close range.
Albion were restricted to shots from distance, with Diomansy Kamara and Robert Koren firing wide.
The second half began at an electrifying pace, as the Baggies stormed into a one goal advantage on 48 minutes - courtesy of Phillips' 13th goal of the campaign.
Rangers weren't behind for long though, with Blackstock heading home at the far post on 62 minutes, just seconds after he entered the fray from the bench.
Hope suddenly sprang eternal and when Furlong was sent crashing to ground inside the box on 65 minutes, a third goal appeared inevitable.
Kiely hadn't read the script though, as he dived full stretch to his left to tip Furlong's spot-kick round the post.
Gera entered the fray soon after and came up with the decisive winner, to leave Rangers embroiled in the scrap for survival.
Buoyed by the victory against Leicester City last time out, John Gregory named an unchanged line-up.
Rohan Ricketts had to make do with a place on the bench after his loan move from Wolverhampton Wanderers, as did the fit-again Blackstock.
It was Rangers' hero of the hour at Leicester a fortnight earlier - Marc Nygaard - that created the R's first chance inside the opening 90 seconds.
The Great Dane's commanding presence spelled panic in the Albion area and when he fed Cook, the livewire winger left two in his wake before thrashing an enticing ball across the face of the six-yard box.
The Baggies were by no means second best though, with Diomansy Kamara firing a yard or so wide with an audacious 25-yard strike that had Lee Camp diving at full stretch to his left.
Long range shooting was certainly the order of the day for the visitors in the opening exchanges.
On nine minutes, Inigo Idiakez's misplaced pass fell at the feet of Robert Koren, whose dipping 30-yard half volley arrowed inches over the bar.
Albion were enjoying the lion's share of possession as the clock ticked towards the 11th minute, with Jason Koumas seeing his deflected free-kick well saved by Camp.
Rangers were forced into a change on 13 minutes, with Steve Lomas replacing Idiakez - who appeared to be the victim of a hamstring injury.
The substitution did little to deter the R's though, with Ainsworth's bustling run ending with him feeding Paul Furlong. Arriving on to the ball at pace, the evergreen striker poked his eight-yard effort a yard or so wide of Dean Kiely's left hand post.
Ainsworth was enjoying a productive afternoon from the right flank and when he pounced on Sam Sodje's weak defensive header and delivered an excellent cross midway through the half, only a vital defensive header from Paul McShane denied Furlong a certain tap-in at the far post.
The R's right winger was in formidable form. On 38 minutes, his out-swinging cross caused yet more panic in the Albion rearguard, from where Sodje produced a magnificent goal-saving header to thwart the on-rushing Furlong.
Referee Mr Armstrong thought otherwise though, awarding a Baggies goal-kick, when it appeared for all the world as though the Reading loanee got the final touch.
The second half was just three minutes old when Albion took the lead.
Moments after Furlong squandered a golden opportunity from yet another Ainsworth cross, the Baggies broke at pace.
Kiely's long throw found Jason Koumas, whose neat, threaded pass ripped open the Rangers defence.
Phillips - with time to turn - left Damion Stewart for dead, before smashing the ball beyond Camp with a powerful right footed strike from 12-yards.
Buoyed by his goal, Phillips forced Camp into a fine one handed save a minute later, before half-time substitute Nathan Ellington did the same from fully 25-yards on 51 minutes.
Undeterred, Rangers created a chance of their own on 56 minutes, with Lomas' rising half volley forcing Kiely to dive full stretch to his left to tip the ball to safety.
Sensing the game was slipping away from his side, Gregory introduced Blackstock at the expense of the tiring Nygaard on the hour.
The substitute made an immediate impact.
Ainsworth yet again proved to be the catalyst, racing to the by-line before pulling the ball back across the face of goal.
Blackstock arrived bang on cue, heading the ball back in the direction from where it came, giving Kiely absolutely no chance.
The R's had vocal appeals for a spot-kick waved away moments later, when Furlong appeared to be dragged to ground.
Referee Mr Armstrong had no qualms about awarding one 60 seconds later though, after Furlong broke into the box and was sent tumbling to ground by a combination of Neil Clement and Paul Robinson.
The same player picked himself up, brushed himself down, only to see his left footed effort saved by Kiely, low to his right.
Despite the missed penalty, Rangers were enjoying a purple patch, with Blackstock heading a yard wide of the far post from Cook's corner kick and Lomas forcing Kiely to gather at the second attempt after his pop-shot from distance.
But it was Albion who bagged the all-important third goal of the contest six minutes from time, when Ellington left Mancienne trailing in his wake and sent an inviting ball to the far post.
Substitute Gera made no mistake, smashing an unstoppable six-yard volley into the roof of the net, to the delight of the 3,000-plus Albion fans.
QPR: Camp, Bignot, Cullip, Bolder, Ainsworth (Ricketts 82), Cook, Stewart, Idiakez (Lomas 13), Furlong, Nygaard (Blackstock 61), Mancienne.
Subs: Cole, Kanyuka.
Scorers: Blackstock 62
Bookings: Blackstock 62, Ainsworth 81, Camp 93
WBA: Kiely, Robinson, Clement, Greening, Kamara (Gera 85), Koren, Carter (Ellington 46), Koumas, McShane, Phillips (Gera 77), Sodje.
Subs: Daniels, Albrechtsen.
Scorers: Phillips 48, Gera 84
Bookings: Gera 84
Referee: Mr P Armstrong
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