Swansea City’s unbeaten away run came to an end as ruthless West Brom finishing sent the Baggies back to the top of the Championship.
The visitors had started brightly enough but found themselves 2-0 down as Semi Ajayi headed in the opener and the excellent Matheus Pereira steered in to double Albion’s lead.
Sam Surridge’s sixth goal of the season had provided hope of a comeback, but when Swansea gifted Hal Robson-Kanu the home side’s third before the break the writing was on the wall.
Matt Phillips scored the fourth from a delightful Pereira cross, as Swansea were carved open on the counter, before Kyle Edwards fired home the fifth to cap a miserable day for the visitors.
It was the first time the Swans had conceded five in a game since a 5-0 defeat at Manchester City in April 2018, and their heaviest defeat at The Hawthorns since 1929.
Head coach Steve Cooper had named an unchanged XI from that which started against Fulham, while Nathan Dyer and Aldo Kalulu returned from injury to be included among the substitutes.
It proved to be an open start, Connor Roberts clearing a Pereira cross from a fast Baggies counter, before Kristoffer Peterson had a header blocked from the full-back’s delivery at the other end.
Surridge proved a nuisance for the home defence and he forced keeper Sam Johnstone into a sharp stop with a rising angled volley from a George Byers pass.
Peterson had started particularly brightly, and one slaloming run took him past three defenders before his low shot found the clutches of Johnstone.
Freddie Woodman was the next keeper in action, tipping over a powerful 19th-minute drive from home skipper Jake Livermore.
The Swansea man then made a superb save to deny Kyle Bartley from close range after the centre-half had been left unmarked at a free-kick.
But there was nothing Woodman could do when a header back across goal allowed Ajayi to convert from the edge of the six-yard box in the 26th minute.
The goal was a little harsh on the Swans, who had given as good as they got during the opening exchanges but they only had themselves to blame for Albion’s second.
The ball was conceded cheaply and the visitors were all at sea as space opened up in their midfield and, an intelligent pull-back from Grady Diangana allowed Pereira to sweep home left-footed.
The Swans needed something before the break, and they got it just three minutes after Pereira’s goal.
Matt Grimes’ deep corner found its way to Surridge, and the striker managed to touch the ball into the corner of the net at the second attempt.
But the Swans almost immediately undid that good work. Again possession was surrendered slackly and Robson-Kanu was played in on and goal and calmly steered the ball into the far corner to restore the hosts two-goal lead.
Cooper sent on Dyer for Peterson at the interval, with Andre Ayew playing through the middle and Surridge heading out on the left wing.
But the hosts remained on the front foot, with Pereira escaping his marker with clever movement but taking too long to get his shot away as the Swansea defence scrambled back.
Just before the hour mark Woodman prevented Kyle Naughton putting through his own net, after an offside from a short West Brom free-kick went unnoticed by the officials.
But the home side made no mistake with 20 minutes to play. A poor Dyer pass stopped a Swansea attack in its tracks and when Grimes then miscontrolled when trying to mop up the danger, Pereira pounced to float a lovely cross for Phillips to head back across goal and find the far corner.
It was soon five, sub Edwards smashing home with his first touch after being teed up by Pereira’s sliding pass to add to Swansea’s misery.
West Brom: Sam Johnstone, Nathan Ferguson, Kyle Bartley (Ahmed Hegazi 66), Semi Ajayi, Kieran Gibbs Darnell Furlong 56), Romaine Sawyers, Jake Livermore (captain), Matt Phillips, Matheus Pereira, Grady Diangana (Kyle Edwards 71), Hal Robson-Kanu.
Substitutes (not used): Jonathan Bond, Filip Krovinovic, Gareth Barry, Charlie Austin.
Swansea City: Freddie Woodman; Connor Roberts, Ben Wilmot, Mike van der Hoorn, Kyle Naughton; Jay Fulton, Matt Grimes (captain); Andre Ayew (Aldo Kalulu 83), George Byers (Barrie McKay 75), Kristoffer Peterson (Nathan Dyer 46); Sam Surridge.
Substitutes (not used): Kristoffer Nordfeldt, Jake Bidwell, Ben Cabango, Tom Carroll.
Referee: Keith Stroud
Attendance: 22, 927