Recent field collections and surveys of herbarium specimens have raised concerns about species circumscriptions and recovered several morphologically distinct populations in Mentzelia section Bartonia (Loasaceae). From the Colorado Plateau, we name M. paradoxensis from Paradox and Gypsum valleys of western Colorado, which is closely related to M. marginata. We name M. holmgreniorum from northeastern Arizona and M. filifolia from the northern border region of Arizona and New Mexico, both of which share morphological similarities with M. lagarosa, M. laciniata, and M. conspicua. From north central New Mexico, we name M. sivinskii, which is most closely related to M. procera and M. integra. We describe three varieties of M. longiloba, including M. longiloba var. yavapaiensis, which is distributed throughout Arizona, M. longiloba var. pinacatensis, which is narrowly distributed in the Pinacate Desert of Sonora, Mexico, and the northern Chihuahuan Desert M. longiloba var. chihuahuaensis. We propose the new combinations M. lagarosa and M. procera to alleviate the polyphyly of M. pumila.