A controversial Robert Green own goal proved the winner as Queens Park Rangers slumped to another defeat to return to the foot of the Premier League.
Baggies captain Chris Brunt fired the visitors ahead with a low shot from distance.
Four minutes after the break West Brom doubled their lead after Green tipped a high ball into his own net, despite being clearly impeded by Marc-Antoine Fortune on the line.
Djibril Cisse's classy finish gave QPR hope, but West Brom held on - and survived a late penalty shout - to claim the points.
Kick-off was delayed by 15 minutes because of heavy traffic around Loftus Road, caused by thousands of Londoners flocking to the nearby Westfield to grab a Boxing Day bargain.
And QPR boss Harry Redknapp will be eyeing the January sales too, as his side created very little in an insipid first half display. Shaun Wright-Phillips’s incisive run and cross for Jamie Mackie, who glanced his header wide, was the only moment of note for the R’s.
West Brom looked comfortable and, although the goal came from nothing, it was always likely given QPR’s calamitous defending. So when Chris Brunt was given space to run into and unleash a venomous low drive into the bottom corner, there was little reaction from Steve Clarke.
QPR looked shaken by the goal and James Morrison could have inflicted further misery on QPR when his long-range effort in first half injury-time whistled just wide.
But West Brom did not have to wait long to double their lead, although it was a goal Redknapp will rightly bemoan.
Anton Ferdinand’s skyward header appeared to be dropping into Green’s arms, but the goalkeeper could only turn it into his own net under illegal pressure from Fortune. Redknapp couldn’t believe it hadn’t been disallowed, and Green continued his protests at the full-time whistle.
QPR reacted well to going behind with Redknapp changing the formation to a 3-5-2, with Junior Hoilett providing a threat down the left. And the hosts gave themselves when Cisse collected Stephane Mbia’s neat through ball, and classily lifted it over the advancing Ben Foster.
An increasingly desperate QPR threw everything at West Brom in search of an equaliser, but fell victim to more bad luck when a clear handball in the box against Liam Ridgewell when unnoticed.
The boos rang around Loftus Road at full-time, with QPR now six points from safety. West Brom move level on points with Everton in fifth.
MAN OF THE MATCH
Jonas Olsson (West Brom): Made countless blocks and organised the back four well in the face of wave after wave of QPR attacks.
QPR: Green 6; Fabio 5, Ferdinand 6, Hill 6, Traore 5; Wright-Phillips 7, Faurlin 6, Mbia 6, Taarabt 6; Mackie 7, Cisse 7. Subs: Diakite 6, Hoilett 7
West Brom: Foster 7; Jones 7, McAuley 7, Olsson 8, Popov 6, Mulumbu 8, Morrison 7, Fortune 6, Gera 6; Brunt 7, Lukaku 7. Subs: Ridgewell 7, Long 6