Abstract
This contribution describes Phyllanthus eremitus, an extremely narrowly endemic new species from an arenitic rock outcrop in Santa Catarina state, southern Brazil. We assess the conservation status of the new species as critically endangered, because only one population is known, containing less than 30 mature individuals, and it is outside any environmental protection area. We provide a detailed morphological description, a distribution map and photographs of the new species. In addition to that, we lectotypify the name P. hyssopifolioides and present the first records of this species in Santa Catarina, which were discovered during field and herbarium work. Finally, we provide an updated identification key to Phyllanthus in Santa Catarina.