A new species, Serpocaulon australe, from the Yungas Montane Forest of Argentina and Bolivia, is described based on a reinterpretation of the name S. gilliesii. The new species is similar to S. lasiopus, but can be distinguished by its rhizome scales with entire margins and acute-acuminate apices, glabrous fronds with few axillary scales in the segments, sori absent in the apex of the segments, and ascending segments. We investigated the taxonomic relationships among these similar species of Serpocaulon using morphoanatomical characters of the sporophyte and ecological preferences. Morphometric analyses, clustering, and principal coordinate analyses (PCoA), were used to scrutinize the forming of hierarchical non-overlapping groups strictly according to degree of morphological similarity. Twenty-four characters were scored for 65 specimens from herbaria (including type material) and for new collections from our fieldwork.We discuss the taxonomic status of S. gilliesii and S. lasiopus and present nomenclature notes for S. gilliesii. Also, some misidentified herbaria collections have led to the discovery of a Serpocaulon species (S. lasiopus), a new record for the Argentinian flora. Finally, a revised key to the pinnatisect species of Serpocaulon in Argentina is provided.