A FOUR-MINUTE second-half Bristol brace ended the jaded Baggies' 13-game unbeaten run on the road as the hosts came from behind to win.
The defeat meant Albion missed the chance to regain second place from Nottingham Forest, who are now a point ahead having played a game more.
The visitors looked comfortable in the first half and took a 25th-minute lead thanks to top scorer Graham Dorrans' 13th goal of the season.
But a rejuvenated Robins side came out firing on all cylinders after the break and levelled through on-loan Wolves striker Chris Iwelumo ten minutes after the restart.
A minute before the hour-mark, Lee Johnson shocked the Baggies by firing the goal that ultimately ended City's seven-game winless streak.
Roman Bednar (back) returned from injury and Youssouf Mulumbu won an instant recall to Roberto Di Matteo's starting XI following a one-game ban.
The men to make way against an unchanged Robins team were Luke Moore and Andy Slory, who both dropped to the bench.
A long throw into the box in the second minute saw the ball break to Robert Koren, who saw his volley blocked.
Chris Brunt pounced on the rebound and spun on the edge of the six-yard area, only to see his goalbound strike snuffed out by Lewin Nyatanga.
A minute later, a Baggies break saw the ball at the dynamic Dorrans' feet and his fierce low shot deflected off Johnson and forced Dean Gerken into a smart low save.
Danny Haynes posed the Baggies' first problem in the fifth minute when he left Joe Mattock for dead down the right and burst into the box before firing into the side-netting at the near post.
Play immediately switched to the other end, with Bednar put clean through on goal.
But the Czech striker took too long to control the pass and was dispossessed by City skipper Louis Carey, who then appeared to foul him on the edge of the area as they challenged for the loose ball.
The hosts pushed forward again in the 15th minute and Bradley Orr let fly with a 25-yard effort that flew just wide of the right-hand post.
Two minutes later, Bednar flicked on Gabriel Tamas' pass for Dorrans, who dallied on the ball and in the end screwed his shot well wide of the right-hand upright from 15 yards.
Liam Fontaine brought down the ball superbly on the edge of the Albion area in the 22nd minute and shook off Gianni Zuiverloon before prodding it through to David Clarkson, who failed to get enough purchase on his shot to trouble Scott Carson.
But seconds later, the Baggies skipper had to be on his toes to dive to his left and hold Iwelumo's header from Orr's right-wing cross.
Back came Albion and, in the 25th minute, they grabbed the lead their early endeavour had deserved.
Bednar fed the ball out to Brunt and sprinted into the six-yard box to get on the end of the Northern Ireland international's centre, only to see Gerken pull off a fine point-blank save.
Moments later, Zuiverloon's pinpoint pass gave Dorrans a sight of goal and the Scot produced a fine finish, flashing the ball in low off the far post from 15 yards.
Bednar did well to bring down Jonas Olsson's ball in the penalty area in the 29th minute and cut inside Nyatanga only to fire a weak left-foot shot at Gerken.
Six minutes before the break, Olsson and Tamas failed to clear the danger and Haynes had a clear sight of goal - but fired into the side-netting from eight yards when he should at least have hit the target.
Mulumbu delivered the ball to Bednar's feet in the box in the 43rd minute and the Baggies hitman swivelled before seeing his shot cannon back off the chest of Nyatanga.
Two minutes into the second half, referee Neil Swarbrick waved away Bristol penalty appeals for handball when the ball struck Tamas from Iwelumo's knockdown. TV replays showed it was the right decision.
The Baggies defence then backed off from two City attacks, which both led to deflected shots for corners.
And from the second Johnson flag-kick from the left, Iwelumo managed to smuggle home a leveller from close range on 55.
Four minutes later, provider Johnson turned scorer as he stole in on Haynes' low pull-back from the right-hand by-line and tapped home to pinch the lead.
Gary Johnson's men continued to press and Fontaine rocketed a 20-yard shot just past the left-hand angle of post and bar in the 63rd minute.
Carson came to Albion's rescue in the 69th minute when he blocked Haynes' ball at the near post after the Robins winger had again burst past Mattock.
As play continued, the ball was delivered to the back post where Iwelumo headed inches over the bar.
In an attempt to at least bag a point from the game, Di Matteo took off right-back Zuiverloon and put on midweek reserves hat-trick hero Chris Wood in attack.
Two minutes from the end, a break saw Tamas find Dorrans and he slid Bednar in behind the City defence, but the former Hearts man drilled his shot straight at Gerken.
Bednar found himself in a similar position in injury time as he drifted inside Orr from the left but his weak shot was easily dealt with by Gerken.
BRISTOL (4-5-1): Gerken; Nyatanga, Carey, Orr, Fontaine; Haynes (Sproule 83), Elliott, Johnson (Hartley 90+4), Skuse, Clarkson (Maynard 79); Iwelumo. Subs not used: Collis (gk), Williams, Plummer, Edwards.
ALBION (4-5-1): Carson; Zuiverloon (Wood 79), Tamas, Olsson, Mattock; Brunt, Koren (Cox 62), Mulumbu, Dorrans, Nouble (Slory 56); Bednar. Subs not used: Kiely (gk), Moore, Meite, Thorne.
GOALS: BRISTOL - Iwelumo (55), Johnson (59). ALBION - Dorrans (25).
BOOKINGS: ALBION - Olsson (foul 49), Brunt (foul 82), Bednar (foul 87).
REFEREE: N Swarbrick (Lancashire).
ATTENDANCE: 14,374 (1,587 Albion fans).
ALBION STARMAN: Youssouf Mulumbu.