Peromyscus fraterculus () is a small rodent commonly called the northern Baja deermouse. Its pelage is dark, the tail is considerably longer than the body, and the number of caudal vertebrae ranges from 30 to 34. The geographic distribution of P. fraterculus includes the area west of the Colorado River, from southern California in the United States of America southward to the Baja California Peninsula, Mexico, south of La Paz Isthmus. It is found in a broad variety of habitats from sea level to 1,175 m. P. fraterculus does not have special conservation status, although the island populations have conservation issues, mainly due to the introduction of non-native species. It is listed as “Least Concern” (LC) by the International Union for Conservation of Nature and Natural Resources.