A shocking return to his former club, manager Roberto Di Matteo, who helped the Baggies to Premier League promotion two seasons ago, after Peter Odemwingie scored the winner in the second half.
Meanhwile, former Chelsea assistant manager, Steve Clarke has had an impressive start to his managerial career with West Brom. With this win, the Baggies claimed fourth spot, after Everton and Tottenham lost their matches on Saturday.
Chelsea are now four points behind league leaders Manchester City and are without a win in four league games including two defeats.
Although the Blues dominated possession and various other statistics, West Brom continued their fine form at home. Clarke's side only dropped three points against Manchester City in seven home games this season.
Chelsea's back-line is worry for Di Matteo, clearly missing the likes of John Terry, who is out for nearly three weeks with a thigh injury, and left-back Ashley Cole was benched after recovering from a hamstring problem.
The likes of Juan Mata, Oscar and Ramires were benched keeping in mind the crucial Champions League away match at Juventus on Tuesday. However, Eden Hazard, scorer of Chelsea's only goal, alongwith Daniel Sturridge, Fernando Torres and Victor Moses couldn't find a way to score.
Torres continued his poor form while Sturridge had many shots either saved or shot wide from goal.
Despite spending nearly 100 million during the transfer window, Chelsea have had a recent slump in form, both in the Premier League and in Europe.
With Juventus next in the Champions League and Manchester City next weekend, can Chelsea return to winning ways?