ALBION came from behind to get back to winning ways thanks to Chris Brunt's deadly second-half double.
The Baggies had edged a first half of few chances but fell behind to Chris Porter's scrambled goal in the 49th minute.
However, Brunt restored parity in the 67th minute with a clinical 20-yard finish and the Northern Ireland international's ninth of the season from close range ten minutes later turned the game on its head.
And sub Simon Cox put the result beyond doubt seven minutes from time by hitting double figures for the term.
This was a gutsy performance from Roberto Di Matteo's troops, who overcame the fatigue of their eighth game in just 24 days to make it only one defeat in nine league fixtures.
New loan signing Ben Watson was handed his debut as Di Matteo made five changes from Wednesday's FA Cup exit.
Jerome Thomas, returning from a five-match ban, Brunt, Jonas Olsson and fit-again Marek Cech were also recalled.
Gianni Zuiverloon and Joe Mattock were left out altogether while Youssouf Mulumbu and Cox dropped to the bench. Roman Bednar missed out with a groin injury.
Derby made three changes, with ex-Baggie Rob Hulse, the injured Chris Commons and suspended Jay McEveley making way for Gilles Sunu, Dean Moxey and Porter.
Both sides got stuck in from the start and it was Albion who threatened first in the sixth minute when Brunt smashed a loose ball wide from 15 yards after forceful play by Thomas.
The Baggies went close again when lone striker Luke Moore saw his near-post flick from Robert Koren's low cross clip Jake Buxton and fly just wide.
Derby, who had beaten Albion's automatic-promotion rivals Newcastle and Nottingham Forest in their last five league games, did not record a shot until the 16th minute when Moxey fired well wide from 20 yards.
Albion threatened again when Olsson headed Brunt's free-kick against the back of Porter in the six-yard box, but the Rams striker reacted quickest to the loose ball and cleared before the vice-skipper could prod home.
The Baggies created the game's first clear-cut chance on 26 when Olsson's defence-splitting pass put Koren clear.
The Slovenia skipper controlled well before sending an acute half-volley just over from 15 yards.
Albion had loud penalty appeals turned away moments later when Moore's goalbound stab from Thomas' low centre appeared to clip Robbie Savage's arm before keeper Stephen Bywater gathered.
The game then descended into a war of attrition and it wasn't until the 42nd minute that either side threatened, when lively Derby winger Stephen Pearson skipped past Brunt and saw his cross volleyed well wide by Buxton.
Pearson caused more problems down the left seconds later and his low centre just evaded Michael Tonge on the edge of the six-yard box.
That proved the final action of a half Albion had shaded - but they fell behind within four minutes of the restart.
Savage curled the ball into the penalty area and as the Baggies defence closed in on Green, the ball broke for Porter who slid home from ten yards.
Albion came within inches of levelling just three minutes later when Brunt towered in the Derby box to head agonisingly wide from Dorrans' pinpoint free-kick.
The game was still lacking any real rhythm as Brunt and Tonge exchanged wayward long-range efforts.
Albion had a lucky escape on the hour-mark when Gilles Sunu blasted a great close-range chance over from Green's pull-back.
But the hosts regrouped and Brunt levelled on 67.
The Northern Ireland international did brilliantly to reach sub Cox's throughball and hold off two recovering Derby defenders before rifling past Bywater from 20 yards.
The equaliser breathed new life into the Baggies' tiring limbs and Bywater had to be alert to deal with a downward header from sub James Morrison from Brunt's corner.
And Albion turned the game on its head on 77 when they surged in front for the first time.
Dorrans' corner created mayhem in the Rams' six-yard box and Brunt was in the right place to prod home in front of the jubilant Smethwick End.
Derby threatened briefly when Porter headed wide from Tonge's left-wing centre.
But Albion, despite their recent fixture glut, finished strongly and Brunt wasn't too far away from completing a hat-trick when he swivelled and let fly from 30 yards.
And Cox put the result beyond doubt on 83 when he beat the offside trap to race onto Morrison's exquisite ball over the top before smashing past the helpless Bywater.
Albion played out the final few minutes with no further alarms to seal one of their gutsiest wins of the season.
ALBION (4-5-1): Carson; Tamas, Meite, Olsson, Cech; Brunt, Koren (Morrison 59), Watson (Mulumbu 81), Dorrans, Thomas; Moore (Cox 59). Subs not used: Kiely (gk), Wood, Slory, Nouble.
DERBY (4-5-1): Bywater; Hunt, Moxey (Pringle 90+4), Barker, Buxton; Sunu (Martin 84), Tonge, Savage, Green, Pearson; Porter (Davies 79). Subs not used: Deeney (gk), Leacock, Hulse, Anderson.
GOALS: ALBION - Brunt (67, 77), Cox (83). DERBY - Porter (49).
BOOKINGS: ALBION - Brunt (foul 33). DERBY - Barker (foul 37), Moxey (foul 76).
REFEREE: P Crossley (Kent).
ALBION STARMAN: Chris Brunt.