Chelsea had conceded only twice since Thomas Tuchel took charge in January. On Saturday, they allowed five goals to one of the lowest-scoring teams in the Premier League.
Thiago Silva's red card in the first half gave West Bromwich Albion the opening they needed en route to a shocking 5-2 result at Stamford Bridge. The loss - Tuchel's first as Chelsea manager - also brings an abrupt end to the Blues' 15-match unbeaten run across all competitions.
It's one to remember for the Baggies, who hadn't won a league match in west London since 1978.
West Ham United can now leapfrog Chelsea into fourth place with a win over Wolverhampton Wanderers on Monday.
Chelsea's day unraveled shortly after Christian Pulisic's opening goal in the 27th minute. Silva, who just returned from injury, was sent off for a second yellow card after a diving challenge on Okay Yokuslu. Then, just before halftime, West Brom's Matheus Pereira delivered a pair of blows in quick succession.
Goalkeeper Sam Johnstone played a long, searching pass that found Perreira in open space in first-half stoppage time, and the Brazilian struck minutes later with a calm and collected left-footed finish.