The genus Piaranthus R. Br. comprises a group of small-stemmed, mat-forming stapeliads occurring in the Cape Province of South Africa and in southern Namibia. Here, they typically can be found under shrubs in karroid vegetation.
Living plants in habitat as well as in cultivation and herbarium material were used in this monograph to study the character state and biology of the closely related species. The comparative morphological analysis of vegetative and generative organs, combined with data from an investigation of floral flavonoids, from a breeding programme and from field studies, form the basis for the new taxonomic approach presented. Of the 16 species formerly recognized, only P. comptus, P. decorus, P. geminatus, P. parvulus, P. framesii and P. punctatus are accepted, supplemented by a new species from the southwestern Cape, P. barrydalensis. Additionally two new combinations are made: P. decorus subsp. cornutus and P. geminatus var. foetidus.