A new mayfly species, Dipteromimus flavipterus sp. nov., of the family Dipteromimidae is described on the basis of specimens of males, females and mature nymphs collected from Ofunato, Iwate Prefecture, Japan. This new Dipteromimus species is characterized by a large body size, a yellowish body color (especially the frontal border of the forewings), short penis lobes, a slight elongation backwards of the female 7th abdominal sternum, and a rounded backward projection of the female 9th abdominal sternum, etc., in comparison with another known dipteromimid mayfly Dipteromimus tipuliformis McLachlan, 1875. Full descriptions of males, females, mature nymphs and eggs of this new species have been provided. We also examined and discussed the genetic relationship of two dipteromimid mayflies to settle the taxonomic status, inferred from the partially sequenced large mitochondrial ribosomal subunit (16S rRNA) genes. Consequently, phenetic and molecular phylogenetic analyses agreed in terms of clustering.