Reynosia vivesiana Trejo (Rhamnaceae) is a previously unrecognized species of woody shrub. It is known from a dry forest in southwestern Puerto Rico, where it has been recorded only in a small area in the Guánica Forest Reserve. The plant was treated for more than seven decades as R. guama Urban, a related species from the Virgin Islands. The new R. vivesiana is separated from R. guama and the other two species of Reynosia Grisebach from Puerto Rico by its glabrous twigs, petioles, and stipels, and by its smaller fruits (ca. 9 mm long), among other features. The segregation of R. vivesiana from R. guama heightens the importance of both species from a conservation standpoint. Based on IUCN Red List criteria, R. vivesiana can be defined as Critically Endangered (CR) and consequently needs legal protection.