A new gall-inducing species of the inquiline tribe Synergini (Hymenoptera: Cynipidae), Synergus itoensis Abe, Ide & Wachi, sp. nov., from Japan is described. The morphological features of the adult clearly indicate that this new species is assigned to the genus Synergus Hartig, 1840, and members of Synergini have all been known as inquilines to date. Field observation showed that adult emergence of this gall wasp from acorns of the evergreen oak, Quercus (Cyclobalanopsis) glauca Thunberg, on the ground is well synchronized with the rapid growth period of fresh acorns in late summer. A rearing experiment demonstrated gall induction by S. itoensis in the seed coat of the acorn of Q. (C.) glauca under field conditions. Moreover, the occurrence of arrhenotoky in this gall wasp and lack of a significant effect of gall induction on acorn size were indicated. Judging from the inconsistency of biological and morphological features in S. itoensis, gall-inducing ability might have been regained within Synergini. The advantages and disadvantages of gall induction in acorns are discussed.