A species of the tribe Synergini (Hymenoptera: Cynipidae) was reared from a gall collected in China on Castanea mollissima Blume and Castanea seguinii Dode. The morphological features of this gall wasp indicate that it is an undescribed species belonging to the genus Synergus and is herein described as a new species, Synergus castaneus Pujade-Villar, Bernardo et Viggiani sp. nov. (Hymenoptera Cynipidae: Synergini). This is the first known instance of a species of Synergus emerging from Castanea galls (Fagaceae). Data on the diagnosis, distribution, and biology of the new species are given. Morphological features of this species are discussed and illustrated. No other known gall-inducing species emerged from the collected galls, whereas tens of specimens of parasitoids and adults of the inquiline micromoth Andrioplecta pulverula Meyrick (Lepidoptera: Tortricidae) did. 28S-D2 and COI sequences confirmed that S. castaneus is a distinct species belonging to the genus Synergus.