The recently resurrected monotypic genus Monrosia (Polygalaceae) is endemic to the Argentinian provinces of Catamarca to San Juan in the southern Andes. We here describe and illustrate Monrosia sanjuanensis, a new species endemic to the Province of San Juan and the second species of the genus, and revise the genus. Monrosia and its two species are described and analysed in a phylogenetic context, using nuclear and chloroplast DNA sequences. A distribution map, illustration, photos of the morphological structures of the two species, and taxonomic key are provided. We designate a lectotype for the name M. pterolopha.