With Norwich City having beaten Millwall 4-3 earlier in the day to go three points clear at the top, Marcelo Bielsa's side required victory to move into pole position.
But it was West Brom which took the spoils from an action-packed encounter, Darren Moore's side – which had hit the woodwork through Jay Rodriguez in the first half – carving the Leeds defence open at will after the interval.
Hal Robson-Kanu got the rout started, slotting home at the culmination of a slick counter-attack after Leeds's Kemar Roofe had a penalty appeal turned down at the other end.
Harvey Barnes squandered a brilliant opportunity to double the hosts' lead shortly after, but the Baggies had their second when Bailey Peacock-Farrell failed to deal with Matt Phillips' speculative effort.
Barnes made up for his earlier miss in the 82nd minute, racing clear before lifting a delicate lob over Peacock-Farrell, who was beaten again a moment later when Dwight Gayle rounded off a brilliant West Brom performance with a clever back-heel, although Pablo Hernandez did head in a stoppage-time consolation.
The victory ends West Brom's four-match winless streak, lifting it up into fifth, while Leeds dropped to third.