Recent collections in the seasonally dry tropical forests of Hispaniola have uncovered a new species of the American clade Castela, described here as Castela plenaensis, endemic to the eastern side of the western extension of the Sierra de Bahoruco in the Dominican Republic just south of the geological formation, Hoyo del Pelempito. Castela plenaensis forms part of the diverse, Greater Antillean Caribbean clade, the most species-rich group among all Castela, and is sister to another recently described Hispaniolan endemic, C. senticosa. We provide a description and illustration of the new species and place it phylogenetically, using morphologic and molecular data. We also provide an identification key to the three species of Castela on Hispaniola and a distribution map of these species. Also, we lectotypify Castela depressa, the type species of the genus, as well as C. erecta. The description of yet another species of Castela from Hispaniola further highlights the importance of the understudied, but yet biodiverse, seasonally dry tropical forests of the Greater Antilles.
