Plagiotrochus masudai Ide, Wachi & Abe sp. nov. (Hymenoptera: Cynipidae), a gall inducer on Quercus (Cyclobalanopsis) glauca Thunberg in Japan, is described. This is the first description of an oak gall wasp species (tribe Cynipini) inducing galls on Cyclobalanopsis. The unique life cycle of P. masudai takes 3 yr to complete. The adult wasps of the sexual generation emerge from bud galls on Q. glauca in spring of the first year, and those of the asexual generation emerge from twig galls in spring of the third year.