The Eastern Palaearctic and Oriental fauna of gall wasp inquilines (Hymenoptera: Cynipidae) has been historically understudied. Previously, ten species of Synergus Hartig (Hymenoptera: Cynipidae: Synergini), the most speciose genus of inquilines associated with Fagaceae galls worldwide, were known to occur in mainland China despite this region harbors a great number of potential fagaceous tree hosts. After the examination of all the Synergus specimens deposited at the Parasitic Hymenoptera Collection of the Agriculture and Forestry University of Zhejiang (ZAFU, China), we provide a revision of this genus in mainland China. Three new species (S. drouarti Pujade-Villar, sp. nov., S. ponsatiae Lobato-Vila & Pujade-Villar, sp. nov. and S. rovirae Lobato-Vila & Pujade-Villar, sp. nov.) are described and illustrated. Four species (S. areolatus (Weld, 1926) comb. nov.; S. formosanus Schwéger & Melika, 2015, S. gifuensis Ashmead, 1904 and S. ishikarii Melika & Schwéger, 2015) are new records for this area. Saphonecrus reticulatus Pujade-Villar, Wang & Guo, 2014 is proposed as syn. nov. of Synergus castaneus Pujade-Villar, Bernardo & Viggiani, 2013, and Synergus changtitangi Melika & Schwéger, 2015 as syn. nov. of S. deqingensis Pujade-Villar, Wang & Chen, 2015. We provide new biological and distributional data for all the species as well as redescriptions, pictures and other corrections for those which were originally poorly characterized. Lastly, we provide an updated key to the Eastern Palaearctic and Oriental Synergus species, with a table summarizing all of them.