A large number of populations of Sternomoera (Crustacea: Amphipoda) are confirmed from epigean waters in Hokkaido and Honshu, Japan. These populations are clearly separated into two groups. One group is identified as S. yezoensis (Uéno, 1933), occurs in the inland of Hokkaido. The other group possesses a wide range of morphological variation and is recorded from the coast of Hokkaido to the coastal and the inland regions of Honshu. Fifty-six populations of the second group are analysed by cluster analysis of quantitative characters. These populations are separated into two strongly-differentiated main clusters. One cluster, confined to the coastal area, is considered to be a new species, S. rhyaca sp. nov. The other cluster, confined to the inland, includes S. japonica (Tattersall, 1922) and S. hayamensis (Stephensen, 1944). Quantitative analysis shows that S. japonica and S. hayamensis are continuously intergraded by intermediate populations, consequently S. hayamensis is downgraded to a junior synonym of S. japonica. A revised diagnosis of the genus, a key to species, descriptions of S. rhyaca sp. nov. and S. japonica, and geographical distribution of Sternomoera spp. are presented. Laboratory observations reveal two types of reproductive behaviours in Sternomoera spp: S. rhyaca sp. nov. and S. japonica are ‘mate-guarders’; S. yezoensis is a ‘non-mate-guarder’. Sternomoera yezoensis possesses sexually dimorphic pleopods, though pleopods of S. rhyaca sp. nov. and S. japonica are monomorphic. The modification in the male pleopods seems to be correlated with the absence of the precopulatory carrying.