Petrosaurus mearnsi, also called commonly the banded rock lizard and Mearns' rock lizard, is a species of lizard in the family Phrynosomatidae. The species is native to western North America.
The specific name, mearnsi, is in honor of American naturalist Edgar Alexander Mearns, who collected the first specimens.
Petrosaurus mearnsi is endemic to extreme southern California and Baja California, Mexico. It also occurs on Isla El Muerto.
Petrosaurus mearnsi is an extremely flat-bodied lizard. Its dorsum is olive, brown or gray, with white or bluish spots. It has a single black collar, a banded tail, and granular scales on its body, with keeled tail and limb scales. Individuals may have a snout-to-vent length (SVL) of . Males have more pronounced throat patterns and brighter blue coloring than females.
Petrosaurus mearnsi is associated with boulder hillsides, extending in Baja California to chaparral and pinyon-juniper woodlands.
Petrosaurus mearnsi is omnivorous, feeding not only on insects and spiders, but also on buds and flowers.
Gravid females of Petrosaurus mearnsi lay eggs from June through August. Clutch size varies from 2 to 6 eggs.