All known specimens of the presumed extinct freshwater shrimp species Syncaris pasadenae, formerly known from the Los Angeles River and other lowland stream sites in southern California, were examined. Existing specimens are housed in the Natural History Museum of Los Angeles County, California Academy of Sciences, or U.S. National Museum of Natural History, Smithsonian Institution, Washington, D.C. Syncaris pasadenae is diagnosed and redescribed and compared to its only congener, Syncaris pacifica, an extant species restricted to a few small coastal streams in northern California (Marin, Napa, and Sonoma counties). Syncaris pacifica is also redescribed. Salient and consistent differences between the species are the dorsal dentition of the rostrum and the number of pereopods bearing exopods, a character often associated with higher-level taxonomic differences in the past. The distribution of the species and the position of the genus within the family Atyidae are discussed.