Ophiocordyceps unituberculata, a new insect pathogenic fungus from southwestern China, is described using molecular phylogenetic and morphological data. This fungus differs from other Ophiocordyceps species by its enormously long monophialidic conidiogenous cells and a large periclinal protuberance often growing near the apex of conidiogenous cells, single conidia (lanceolate to fusiform) embedded in mucous sheath, and much larger conidial dimensions. Four-locus (nrSSU, nrLSU, tef-1α and rpb1) and ITS data phylogenetic analyses show that O. unituberculata belongs to the Hirsutella nodulosa clade within the genus Ophiocordyceps of Ophiocordycipitaceae and is a separate clade from other allied species. Molecular phylogeny and morphology both strongly support the distinctiveness of this taxon. The interspecific relationships in the H. nodulosa clade are discussed.