A new freshwater opecoelid species, Sphaerostoma tamarae sp. nov., is described from Rhynchocypris percnura mantschurica caught in the Syumnyur River (Amur River basin, Russia). This parasite species differs from all other congeners in the distribution of the vitelline follicles (restricted to the hindbody) and the morphology of the ventral sucker (strongly protuberant). Molecular phylogenetic analysis based on 28S rRNA gene sequences and the morphological analysis allow to tentatively assign the new species to the genus Sphaerostoma, without closing the question regarding the true genus affiliation of this species. In addition, the authors provide an argument for renaming the Plagioporinae to the Sphaerostomatinae.