A pyraustine species of the genus Ostrinia Hübner collected at Shigakogen, central Honshu, Japan is described as new to science under the name of Ostrinia ovalipennis sp. nov. This species is morphologically similar to the Far Eastern knotweed borer, Ostrinia latipennis, but is distinguishable from O. latipennis in wing marking and male genitalia. Larvae of O. ovalipennis feed on the knotweed, Reynoutria sachalinensis (Polygonaceae), as do larvae of O. latipennis. In Shigakogen, adults of the two knotweed-boring species co-occur in the same season. Adults of O. ovalipennis were captured only in July, suggesting that this species is univoltine. The fact that O. ovalipennis shares several morphological and ecological features with O. latipennis suggests that the species is the closest relative of O. latipennis.