The squirrel was collected during routine sampling and testing.
Riverside County officials said it was the first test of an animal in the county that showed signs of the plague in nearly a decade.
The ground squirrel was found at the Fern Basin campground north of Idyllwild on September 6 during routine testing.
The risk of contracting Bubonic plague is low, health officials said. Residents, campers and hikers are advised to avoid contact with rodents and other wild animals; contact with pets is also discouraged.
Symptoms of Bubonic plague include sudden onset of fever, chills and tender, swollen lymph nodes.
Health officials advise anyone with symptoms or who has been in a known plague area to contact their doctor immediately.