Despite an even start, four goals in 19 first-half minutes condemned John Mousinho’s men to a seventh-straight defeat on the road.
The Blues boss had rested five of his players and West Brom gleefully took advantage, albeit Nicolas Schmid gifting them the opening two goals with handling errors.
Thomas Waddingham came off the bench and marked his debut with a goal deep in stoppage time for a rare moment of joy for those travelling supporters as it finished 5-1.
And here are our Pompey player ratings...
1. Nicolas Schmid - 7
Great finger-tipped save to push Johnston’s 55th-minute shot over the bar but otherwise had barely anything to do other than playing the ball out from the back. Solid as ever. Photo: Graham Hunt
2. Terry Devlin - 9
Although the second half petered out, Devlin got better and better as the match wore on. Such a ferocious competitor, always on the front foot and an absolute pain in the backside for West Brom down the right flank. Photo: Graham Hunt
3. Zak Swanson - 8
Back to left-back after his central defensive stint against Southampton and slotted in with the minimum of fuss. Some sloppy moments in the first half, yet always a brilliant competitor and enjoying form of his Pompey life.. Photo: Graham Hunt
4. Regan Poole - 8
Returned from injury to resume centre-half duties and was at his tigerish best. Absolutely dominant and bare put a foot wrong. Photo: Graham Hunt