Two species of the heptageniid mayfly genus Thalerosphyrus Eaton, T. vietnamensis (Dang) and T. separatus n. sp., are described from Vietnam. Thalerosphyrus separatus can be distinguished by the basally detached posterolateral expansions of pronotum in larva and by the Y -shaped penes in male adult. Their larval and adult stages are described with line-drawings of diagnostic characters. Their diagnoses, distributions, habitat and ecological data, and taxonomic remarks are provided.