The infected squirrel was discovered at a Fern Basin campground last month during routine testing by county workers, The (Riverside) Press-Enterprise reported.
It was the first time in about a decade that a test for plague came up positive.
The campground remains open but warning signs have been posted.
"We normally only close the campground if there is a high level of antibodies in the animal and a high level of fleas," said Dottie Merki, chief of the county's environmental health program.
Merki said both the flea count and the antibody count on the infected squirrel were low. However, further testing is under way.
Plague was first introduced into the United States in 1900 by rat-infested ships, most of them originating in Asia.
It is caused by the Yersinia pestis bacteria.