A new species of the genus Chrysonotomyia Ashmead, Chrysonotomyia susbellisp. nov., is described from the Rice University campus in Houston, Texas, USA. The species is a parasitoid emerging from Neuroterus nr. bussae galls in leaves of the southern live oak (Quercus virginiana). This represents the 6th species described from North America north of Mexico and the first in the world known to parasitize cynipid gall wasps. This discovery hints at an entire undiscovered niche between Chrysonotomyia parasitoids, cynipid gall wasps, and oaks in the Nearctic, which is a global biodiversity hotspot for oaks and cynipids. This new species description is complemented by mtDNA-COI-barcode data and information on the natural history of this species. We record host association, phenology, and report a leaf-scanning behavior performed by females, presumably to search for host galls. Modifications to the key of New World members of the genus (Hansson 2004) are included to integrate this new species.