We describe a new species of egg parasitoid, Oobius primorskyensis Yao & Duan n. sp., collected from the emerald ash borer Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) near Vladivostok, Russia. Morphological and molecular analyses indicate that this taxon is sufficiently distinct from Oobius agriliZhang & Huang 2005 to merit species status. Furthermore, DNA sequence divergence between O. primorskyensis and specimens in culture of Chinese origin suggest that O. primorskyensis is molecularly distinct from Oobius previously imported into the United Stated and released as biological control agents of emerald ash borer. Laboratory observations showed that O. primorskyensis also differs from O. agrili in diapause pattern. When reared with A. planipennis eggs at 25 ± 1.5° C, 65 ± 10% relative humidity, and a photoperiod of 16:8 (L:D) h, nearly 100% of O. primorskyensis diapaused at the second and third generations after their parents had diapaused, whereas a majority (60–96%) of O. agrili did not diapauses for 11 sequential generations. Diagnosis of the species with respect to O. agrili is provided. The new species is a natural enemy of A. planipennis in its native range and may be considered for introduction against A. planipennis in North America.