The taxonomic history, defining characters, and relationships of the genus Mortoniodendron are briefly sketched. Characters that define M. pentagonum, a species known from a limited area in Guatemala, are elaborated. A closely related but distinct species from Veracruz, Mexico, M. uxpanapense Dorr & T. Wendt, is described and illustrated. The new species is considered to be Critically Endangered (CR) under the IUCN red list classification.