Philonix nigra Gillette, 1889 (Hymenoptera, Cynipidae, Cynipini) induces novel structures, plant galls, on white oaks (Fagaceae: Quercus alba L. and possibly other species classified in Quercus sect. Quercus). Such structures are often inhabited by other cynipids, who apparently cannot induce their own galls on these plants. These inquilinous species are primarily classified in the tribes Synergini (Synergus spp.) and Ceroptresini (Ceroptres spp.) Extensive rearing pf P. nigra galls has yielded numerous associates, including a new species of Synergus.We describe Synergus nigrus Montelongo & Nastasi sp. nov., a new species of inquilinous cynipid discovered from the eastern USA in Philonix nigra galls. This species is morphologically diagnosable, being the first Synergus with a partially smooth mesopleuron and 15 antennomeres in females. This diagnosis was also supported by DNA barcode data (mitochondrial cytochrome c oxidase I gene fragment), when compared to other available Synergini barcode sequences. We discuss key aspects that separate this species from related species and issues with taxonomy of this genus.