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.