Relegation-threatened West Brom put five goals past Edouard Mendy in goal for Chelsea to end the London side’s unbeaten run under the managerial reign of Thomas Tuchel.
Chelsea opened the scoring through Christian Pulisic who pounced on a Marcos Alonso’s rebounded freekick.
Chelsea were reduced to ten men after returnee Thiago Silva was handed a red card for a foul in the 29th minute of the encounter.
West Brom immediately capitalized on the situation with two quick fire goals in first half added time by Matheus Pereira.
Callum Robinson and Mbaye Diagne scored the third and fourth goals in the 63rd and 68th minutes of the game.
Substitute, Mason Mount reduced the deficit for Chelsea in the 71st minute while Callum Robinson buried the game for his side in added time.