A new species, Oxypetalum marchesii, from rocky grassland environments of Uruguay, is here described, illustrated, and compared to morphologically similar species. Data on its habitat, distribution, ecology, and conservation status are also provided. Based on its cpDNA, O. marchesii clearly corresponds to the recently re-circumscribed and morphologically diverse genus Oxypetalum. This species is mostly characterized by its creeping, non-twining habit, widely ovate and shortly petiolate hirsute leaves, relatively large flowers with pale greyish corollas, and short stylar-head appendage that is included within a corona formed of oblong-lanceolate, bifurcate lobes. The conservation status of this species is considered vulnerable because of its small geographic range and restricted population size.