A new species of Merostachys from the Bolivian Yungas, Merostachys yungasensis, is described and illustrated. Merostachys yungasensis is compared to other taxa distributed in Argentina, Bolivia, Paraguay, and Peru, in a table based on morphological characters. Additional micromorphological characters of foliage leaf blade and cauline epidermis are included. In Argentina two species are confirmed: M. claussenii and M. multiramea; the latter species constitutes a new report for this country and is excluded from Paraguay. The specific epithet of M. claussenii is emended, and a lectotype is here designated. Two new synonyms are recognized for M. claussenii and its presence is confirmed from Paraguay after an analysis of previous reports based on misidentifications. The taxa are illustrated. A key for the identification of the species, based on vegetative and reproductive characters, is also provided.