Carex phleoides Cav. is the largest species of the former genus Uncinia Pers. It is widely distributed in the Neotropics and is also known from the Juan Fernández archipelago off the coast of Chile. A second taxon, U. trichocarpa C. A. Mey., is often considered a synonym of C. phleoides, but also sometimes a variety of that species or even a distinct species. We performed a morphometric study using representative material covering the entire distribution and morphological variation of C. phleoides s.l. to clarify its taxonomy. Our analyses revealed that U. trichocarpa is distinct from C. phleoides. Accordingly, we coined the name C. quinquin Jim.-Mejías & Dorr to accommodate this species under Carex L. because combinations of other available names are all blocked. Carex phleoides displayed two extremes of variation with geographical correspondence, a southern form in the Southern Cone north to Bolivia, and a northern one ranging from Bolivia to Mexico. We propose to recognize two subspecies within C. phleoides, the nominate subspecies being the southern one, and the northern one based on the epithet koyamae. Plants from the Juan Fernández archipelago were shown to be morphologically intermediate with the endemic C. plurinervata J. R. Starr, a result that might point to introgression between these two species. We selected lectotypes for nine names, including two of the accepted ones, as well as their synonyms.