It has long been assumed that members of the cynipid tribe Synergini (sensu stricto) are all inquilines, that is, gallers of galls: the larvae can modify the galls of other species but cannot initiate gall formation on their own. Surprisingly, Abe et al. recently showed that one member of the tribe, Synergus itoensis Abe, Ide et Wachi, is a true gall inducer: it produces small galls inside acorns without the help of other species. Here, we present a comprehensive phylogenetic analysis of the Synergini to determine whether S. itoensis acquired this ability recently or if it represents a survivor of ancient gall inducers from which inquiline Synergini evolved. We studied 71 species belonging to five genera, covering all the Palearctic inquiline genera associated with oaks and one outgroup, Rhoophilus loewi Mayr, which is an inquiline in Lepidoptera galls on Searsia (formerly Rhus) (Anacardiaceae). We obtained partial sequences of cytochrome c oxidase subunit I and 28S ribosomal RNA (1.1 kbp of data). The results show that S. itoensis is most closely related to two undescribed species, which have also been reared from acorns. Their life history is unknown but the molecular phylogenetic data and the similarity in gall and adult morphology suggest that they are also gall inducers.The three species are deeply nested within lineages known to be inquilines, strongly suggesting that they acquired the ability to induce galls recently. Based on the molecular phylogenetic data and morphological features, Synergus yukawai (Wachi, Ide et Abe) is transferred back to Saphonecrus (Saphonecrus yukawaicomb. rev.), where it was originally described.