Preston dented West Brom's hopes of automatic promotion with victory in a scrappy Championship encounter.
Emil Riis Jakobsen poked Preston in front from almost on the goalline after Greg Cunningham's ball into the box took a deflection off Kyle Bartley.
Debutant Cameron Archer headed in the second from close range after David Button parried Cunningham's cross.
Albion failed to register a shot on target as they lost at home in the league for the first time this season.
But, for delighted Preston boss Ryan Lowe, who went to joyously salute the travelling fans at the end, it was a great night as his side bounced back from their first defeat under him at Swansea on Saturday.
To add to an already unsettled mood of fans' discontent at The Hawthorns, Albion went into the game without England goalkeeper Sam Johnstone.
He was due back after a three-game ban but he was left out over an "internal issue" by under-fire Baggies boss Valerien Ismael, who also revealed that he will not be considered for selection for Saturday's trip to Millwall.
The taking of all three points - Preston's first at The Hawthorns since the days of only two points for a win in the old Division Two back in September 1973 - was built round a fine hard-working, defensive performance which lifted them two places in the table to 13th.
Albion had rallied late on against Peterborough on Saturday to score three times in the last 11 minutes for a 3-0 win.
But, once on-loan Aston Villa striker Archer had marked his debut with his first league goal just 12 minutes after coming on as a second-half substitute, there never looked likely to be a repeat of that sort of finale from a limp Albion attack, badly missing injured New Year's Day signing Daryl Dike.
By contrast, for Archer, it was an 11th goal of the season, having already scored four for Villa in the Carabao Cup and six for Villa Under-21s in the Papa John's Trophy - and they have all come away from home.
He will now be expected to make his home debut on Saturday when North End host Bristol City.
West Bromwich Albion boss Valerien Ismael told BBC Radio WM:
"We deserve all the criticism for that. It was a poor team performance. I agree with all the boos.
"We have no excuses. We were never in the game. We deserved to lose. When you put in a performance like that, you have to accept it.
"We didn't put in the energy or effort to win that game."
On Sam Johnstone's absence:
"We had an issue internally. It is solved now but that is the reason he wasn't in the squad and won't be on Saturday, but after that he will be back."
Preston boss Ryan Lowe told BBC Radio Lancashire:
"It's been coming. Some of our performances have been 60%. That was definitely 70-75%. I made a few changes to make us more solid and resolute and we got our just rewards.
"We had a game plan and the lads stuck to it fantastically well. We knew what we had to do in terms of silencing their crowd. You can't fear these sorts of sides. You have to go toe-to-toe with them.
"Cameron Archer is a goalscorer. We know what he give us. He gets in the right positions at the right times and, if he continues to do that, he will score more goals."