Matt Phillips ended Leicester's 20-game unbeaten home league run as West Brom clinched a surprise 2-1 win at the King Power Stadium.
The winger cashed in on Danny Drinkwater's error as the Baggies deservedly became the first side to beat the champions at home in the Premier League since Arsenal in September 2015.
Islam Slimani quickly cancelled out James Morrison's second-half opener but Phillips' goal ended Albion's five-match winless streak.
Leicester's focus on the Champions League - where they need just a point from their final two Group G games to reach the last 16 - has impacted on their domestic form and the Foxes were again lacklustre.