Agastoroxenia panamensis Nieves-Aldrey & Medianero, a new genus and species of inquiline oak gall wasps (Hymenoptera: Cynipidae: Synergini), is described from material reared from unidentified Andricus galls (Cynipini) on Quercus lancifolia Schltdl. & Cham. collected in Panama. The new genus is the first genus of inquiline oak gall wasps described from the Neotropics and the first species of this group recorded from Panama. The new genus is related to Saphonecrus and Synergus but differs from these genera mainly by the unique conformation of the male and female antenna.