Striker Marlon King broke his six-game duck and propelled Nottingham Forest into the FA Cup fourth round after a second-half penalty sank West Brom at the City Ground.
King, a £950,000 buy from Gillingham, had failed to score since arriving in November.
But the 23-year-old held his nerve and converted from 12 yards after Geoff Horsfield manhandled Matthieu Louis-Jean in the box, leaving goalkeeper Russell Hoult with no chance.
Their victory drew a line under Forest's 11-game winless streak but misfiring Albion, chasing promotion to the Premiership, saw their own sequence without a win extended into a fifth game.
The early running had been made by West Brom midfielder Jason Koumas, who was first to test Forest goalkeeper Darren Ward after a neat link-up involving Neil Clement and Horsfield ended with a snapshot in the ninth minute from the left.
But another 10 minutes elapsed before Albion's next chance, this time Clement ranging forward and launching a fierce low drive which fizzed marginally wide.
Forest's only first-half attack of note arrived courtesy of King, although his effort was blocked by Clement on the edge of the box.
However, after a pedestrian opening 45 minutes Forest manager Paul Hart sent his side out for the second half with renewed vigour and their reward was not long in coming.
Panic had reigned in the West Brom penalty box when, in the 52nd minute, Louis-Jean chased an apparent lost cause after Gareth Williams' 18 yard effort had been deflected for what had appeared a certain corner.
But after the Frenchman had beaten the odds to keep the ball in play, Horsfield was punished for hauling him down and referee Alan Kaye had no hesitation in awarding a penalty with the striker also earning a yellow card.
Up stepped King, who had made his Forest debut as a substitute against West Brom, to thump the ball low to goalkeeper Hoult's left.
Moments later Forest would have doubled their advantage but for Hoult's stupendous point-blank save which denied a Gareth Taylor volley.
As the Baggies continued to reel, an unmarked King saw his cushioned header plucked out of the air by Hoult following a 63rd-minute cross by Gregor Robertson.
Albion were lacking the penetration to regain their foothold in the match and the Forest resolve held firm as West Brom pressed late in the game.
Ugly scenes ensued when James O'Connor shoved a Forest ballboy in the chest after the youngster hid the ball behind his back instead of releasing it for a throw in.
Teams
Nottm Forest Ward, Louis-Jean, Walker, Morgan, Robertson,
Gunnarsson, Thompson, Williams, Reid, King, Taylor.
Subs Not Used: Jess, Roche, Gardner, Sonner, Doig.
Booked: King.
Goals: King 54 pen.
West Brom Hoult, Haas, Gaardsoe, Moore, Clement,
Sakiri (Dyer 66), O'Connor, Koumas, Robinson, Hulse,
Horsfield (Dobie 67).
Subs Not Used: Murphy, N'Dour, Gilchrist.
Booked: Horsfield, O'Connor, Haas, Moore.
Att: 11,843
Ref: A Kaye (W Yorkshire).
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