| Tóm tắt: | Background and Aims: Graptopetalum is an American genus with 22 species, which has its main distribution in Mexico. From collections made in the municipality of San Juan Teita, district of Tlaxiaco, Oaxaca, a new species for science of this genus was located. Hence, the objectives were to describe and illustrate it, and compare its morphology with G. macdougallii, which is considered its most closely related species. In addition, a map of the distribution of both species is presented, and a lectotype is proposed for G. macdougallii.Methods: Based on collections, review of specialized literature and study of physical and virtual herbarium specimens, a new species of Graptopetalum is proposed. Its morphological description, an illustration, and a distribution map were elaborated, and it was compared with the species that presents the greatest similarity. Its conservation status was evaluated according to the categories and criteria of the IUCN Red List.Key results: Graptopetalum irmasoniae, species of the subgenus Graptopetalum, section Graptopetalum, series Stoloniferae, is described and illustrated. It shows similarities with G. macdougallii, from which it differs by the larger diameter of the rosette, length of the stolons, number, size and shape of the leaves and inflorescence, as well as by the length of the corolla segments and the type of habitat. The new species is Critically Endangered (CR) because of its limited distribution, low population numbers and habitat fragility. The distribution map of both species is presented, and G. macdougallii is lectotypified.Conclusions: Graptopetalum irmasoniae is a microendemic species of the state of Oaxaca, Mexico, where it grows on gypsum outcrops. It is the fourth species of the family in Mexico that is described from this type of soil and given the fragility of its habitat, special attention is required to preserve it in the short term.
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