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We describe Mentzelia canyonensis, a new species endemic to the Grand Canyon. Based on floral and fruit forms, shoot system architecture, and phylogenetic analyses of nuclear ribosomal DNA, we infer that M. canyonensis belongs to the ’subshrubby’ clade of Mentzelia section Bartonia, which includes the morphologically similar M. hualapaiensis, M. oreophila, M. puberula, and M. tiehmii. The new species is most similar to M. hualapaiensis, which is also known only from the Grand Canyon. These two species share leaf shape, subshrubby habits, corolla color, and fruit shape, although they differ in staminode presence and especially in leaf and corolla size. Principal component analyses of variation among floral and vegetative traits demonstrates that M. canyonensis and M. hualapaiensis share highly overlapping floral character space, but M. canyonensis occupies unique vegetative character space compared to similar species. A dichotomous key is provided to assist with identification of, and differentiation among, species of Mentzelia section Bartonia that occur in the Grand Canyon region.