We conducted a study of Prosartes parvifolia S. Watson, a rare Siskiyou Mountains endemic, currently known from only 15 sites in Del Norte Co., California, and Curry and Josephine counties, Oregon. We found that P. parvifolia is (a) fertile, (b) probably not of hybrid origin, and (c) distinct and worthy of recognition as a species. Unlike congeners, its flowers produce ovaries with a single locule, and are pollinated by bees that buzz pollen from connivent anthers. Nectar is not produced. We provide an expanded description, illustrations, and distribution map for P. parvifolia as well as a key to the Prosartes of northwestern California and southwestern Oregon.