The Caribbean islands are a well-known biodiversity hotspot of global importance. Hispaniola (Dominican Republic and Haiti) is a highly diverse island with 42% of its species endemic. Here we describe the new monotypic genus Tainus (Rubiaceae, Rondeletieae), which is endemic to the Sierra de Bahoruco-La Selle. This taxon was originally published as Rondeletia pitreana, but its taxonomic status has been doubtful. We performed phylogenetic analyses with three chloroplast regions (petD, trnK-matK and trnL-F) in order to test the relationships of R. pitreana to other species of Rondeletia and allied genera. Our molecular phylogeny underscores that R. pitreana does not belong to Rondeletia, but has an isolated position within Rondeletieae, whereas Acrosynanthus is sister to an assemblage of Mazaea, Phyllomelia, Rachicallis, Roigella and Rondeletia. The recognition of the genus Tainus is supported by molecular (simple sequence repeats) and morphological characters, among which the most conspicuous is the densely papillate ornamentation of the corolla mouth and lobes. Our findings reveal another case of endemism at a deeper phylogenetic level in flowering plants on Hispaniola. Furthermore, they emphasize the importance of exploring, collecting and performing phylogenetic analyses integrating samples from larger geographical areas across countries as a basis to support nature conservation.