Diastrophus japonicus Wachi, Ide & Abe sp. nov., a gall inducer on Rubus palmatus Thunberg, is described from Japan. This is the first reliable record of Diastrophus Hartig, 1840 (Hymenoptera: Cynipidae: Aylacini) from the Eastern Palearctic region. This article reveals that the genus Diastrophus is widely distributed from the Western Palearctic to the Nearctic regions through the Eastern Palearctic region. The causal process for the extant geographic distribution of D. japonicus was the opening of the Sea of Japan and dispersal during the Pleistocene. The phylogenetic relationships among the members of Diastrophus (D. japonicus and the species considered by a previous study) were inferred based on adult morphological and gall characters, but low consistency indices suggested that the characters exhibited homoplasy.