Background: Mentzelia (Loasaceae) is a genus of approximately 95 species that are largely distributed in western North America; however, much ambiguity remains regarding species in Mexico. Questions: What species of Mentzelia occur in Mexico and how can they be distinguished? Study species: Mentzelia Methods: Fieldwork, herbarium studies and scanning electron microscopy were carried out to determine the diversity of Mentzelia species in Mexico. Results: Twenty-five species of Mentzelia occur in Mexico, of which four taxa are endemic to the country. Five of the six sections of Mentzelia occur in Mexico. Mentzelia section Mentzelia was the most species rich in Mexico (8 spp.), followed by section Trachyphytum (7 spp.), section Bartonia (6 spp.), section Bicuspidaria (3 spp.), and section Dendromentzelia (1 sp.). The sections have different distribution patterns, with some restricted to few areas and one widespread across most of Mexico. Conclusions: This study is the first treatment of Mentzelia that encompasses all species and regions of Mexico, which includes approximately 26 % of the worldwide Mentzelia species. In-depth studies of the species in the region are needed to abate gaps in our knowledge on the extent of species distributions and to clarify species boundaries among some problematic species complexes.