Peter Odemwingie’s second-half header gave Albion another memorable afternoon as their dream season continued with victory over the European Champions.
Shane Long opened the scoring then crossed for Odemwingie’s winner as Chelsea were beaten despite a deflected equaliser by Edin Hazard just before half-time.
It meant a miserable return to The Hawthorns for former Albion boss Roberto Di Matteo as he watched his former club go fourth in the Premier League.
But the Baggies rode their luck as the visitors had two strong penalty shouts rejected and stand-in keeper Boaz Myhill made three fine saves.
Albion survived a double-scare on seven minutes when Liam Ridgewell cleared a Ryan Bertrand shot off the line and then Jonas Olsson was fortunate not to concede a penalty when he appeared to block Edin Hazard’s follow-up with his upper arm.
But the Baggies went ahead three minutes later through a well-worked goal that came from their left flank.
A neat reverse pass from Zoltan Gera found James Morrison in space and his cross was headed in by Shane Long, who was at full-stretch to get the better of David Luiz.
It could have been 2-0 just after the half-hour when Billy Jones raced down the right and crossed towards Long, who diverted the ball on target in a tangle with Gary Cahill but saw it saved easily by Petr Cech.
And the Irishman’s subsequent appeals for a penalty fell on deaf ears.
He went close again 10 minutes before the break when he chased down a crossfield ball from Morrison and cut inside before blasting just wide.
But it was 1-0 on 39 minutes when Cesar Azpilicueta checked onto his left foot and crossed from the right to the far post, where Hazard saw his header take a crucial deflection off Billy Jones to beat keeper Boaz Myhill.
Myhill kept the scores level three minutes into the second half when he pulled off a smart point-blank save to keep out a shot from Daniel Sturridge after a good run and pull-back by Victor Moses.
And the Baggies cashed in on 50 minutes when patient build-up on the edge of the box ended with Long’s perfect near-post cross glanced in by Odemwingie to make it 2-1.
They escaped another strong penalty shout just after the hour-mark when Moses appeared to be barged over by Yacob, but referee Michael Oliver dismissed the appeals.
Myhill excelled again when he pulled off a superb save to keep out a shot on the turn by Sturridge.
And he pulled off an even better save to keep Sturridge at bay when the Chelsea youngster raced through the middle onto a Juan Mata through-ball.
Albion had a chance to extend their lead when Graham Dorrans won a tackle to spark a counter-attack, but the Scotsman could not control a Shane Long cross.
Chelsea piled on the pressure late on and Sturridge dragged a shot just wide with three minutes remaining.
By Steve Madeley