Dryocosmus kunugiphagus Ide and Abe, n. sp. associated with a section Cerris oak, Quercus acutissima Carruth., is described in Japan and Korea. Although section Cerris oaks are the main host plants of Western Palearctic species of Dryocosmus Giraud, hitherto known Eastern Palearctic ones exhibit host specificity with plants of the genus Castanea Mill. and the subgenus Cyclobalanopsis (Oerst.) of the genus Quercus L. Therefore, the present study served as the first record of a Dryocosmus species being associated with section Cerris oaks in the Eastern Palearctic region. The species was previously identified as Spathegaster sp., Andricus inflator Hartig, and Neuroterus boni-henrici Dettmer in Japan; however, examination of adults that emerged from galls resulted in the description of a new Dryocosmus species. Collection and emergence dates indicated the occurrence of a prolonged diapause in the asexual generation of D. kunugiphagus.