A new Neotropical genus of Commelinaceae, Plowmanianthus, is described with five new species. Karyological and morphological data, as well as results of phylogenetic studies, support its inclusion in the subtribe Dichorisandrinae (tribe Tradescantieae) with four other genera (Cochliostema, Dichorisandra, Geogenanthus, and Siderasis). The occurrence of moniliform hairs along the distal margins of the petals is evidence of an even closer relationship with Cochliostema and Geogenanthus, this character being restricted in the family to these three genera. Plowmanianthus may be uniquely characterized as comprising small, perennial rosette herbs with usually one-cymed, axillary inflorescences borne among the lower leaves, an androecium reduced to three fertile stamens, annular, papillate stigmas, and uniseriate to partially biseriate seeds. Plants of the genus are shallowly rooted, the roots not penetrating deeper than the leaf-litter or humus-rich layers of their primary rainforest habitats. Plowmanianthus is also distinctive among the Commelinaceae in the unusually high frequency of cleistogamy in the genus, with some species for which only cleistogamous flowers are known. As currently understood, two species (P. panamensis and P. dressleri) are restricted to the Isthmus of Panama, whereas the remaining four taxa (P. grandifolius subsp. grandifolius, P. grandifolius subsp. robustus, P. perforans, and P. peruvianus) are restricted to lowland Amazonia.