Albion tasted only their second defeat in 10 Sky Bet Championship away matches as they were undone 4-0 by Leeds United at Elland Road.
Pablo Hernandez's 20-yard strike after just 16 seconds put the hosts in command early on before Patrick Bamford doubled their advantage just before the half-hour mark.
Bamford added a deflected third after 63 minutes and Ezgjan Alioski scored in stoppage time as the Baggies, who had won eight of their previous nine matches on the road, tasted Friday-night defeat amid a fervent atmosphere in West Yorkshire.
Albion, who triumphed 4-1 over Leeds in the reverse fixture earlier this season, were shut out for the first time in 10 away games stretching back to early November.
They will now be looking to get back on track when Ipswich Town visit The Hawthorns next Saturday.
Head coach Darren Moore was forced into two changes from the defeat to Sheffield United, Tosin Adarabioyo and fit-again Hal Robson-Kanu coming in for Kieran Gibbs and Matt Phillips.
Yet the visitors got off to a nightmare start as United opened the scoring with their first attack from kick-off inside a minute, Hernandez finding the top corner with a long-range strike.
Sam Johnstone saved well from former Albion forward Tyler Roberts before making another stop to keep out Kalvin Phillips' strike from distance.
However, he could not prevent the hosts doubling their advantage in the 28th minute, Roberts' flick playing in Bamford and he slotted across goal into the corner of the net.
Jay Rodriguez's sweet 35-yard strike forced Kiko Casilla to tip over the crossbar but it was the Whites who held a two-goal advantage at the interval.
Robson-Kanu flicked captain Gareth Barry's inswinging free-kick narrowly over at the start of the second half.
Johnstone had to be alert to get down quickly and deny Mateusz Klich just before the hour mark.
But it was not long before Leeds added a third, Roberts breaking from midfield and finding Bamford, whose shot took a slight deflection off the unfortunate Craig Dawson before wrong-footing Johnstone.
Ezgjan Alioski then added a fourth in added time as the home side ran out winners in Albion's first visit to Leeds since 2007.
Albion: Johnstone, Holgate, Dawson, Hegazi, Adarabioyo, Harper (Field 65), Barry (c) (Morrison 63), Livermore, Robson-Kanu (Montero 70), Rodriguez, Gayle.
Subs: Bond, Townsend, Bartley, Murphy.