Thomas Tuchel's side had initially drawn first blood through Christian Pulisic, but Thiago Silva was sent off before the 30-minute mark for two bookable offences, and the floodgates opened soon after as Matheus Pereira struck twice just before the break.
Callum Robinson notched up a brace of his own while Mbaye Diagne also got in on the act in the second 45, as Tuchel suffered his first defeat since taking the reins at Chelsea, and in humiliating fashion too.
Having also shipped three goals to the Baggies in a 3-3 draw at The Hawthorns back in September, the Blues have set an unwanted club record for the most goals conceded against a single opponent in a Premier League campaign.
Furthermore, the Blues saw their defence breached five times at Stamford Bridge for just the second time in the Premier League - nearly 10 years on from their 5-3 loss to Arsenal in the 2011-12 campaign - and Tuchel was yet to see his side concede a goal on home soil before Saturday's humbling defeat.
Despite their painful loss to Sam Allardyce's men, Chelsea remain in the top four of the table, but West Ham United could leapfrog them if they beat Wolverhampton Wanderers on Monday night. body check tags ::