A captive-born, 13-yr-old female orange-spot freshwater stingray, (Potamotrygon motoro), presented with an acute caudodorsal swelling. Ultrasonography revealed an intracoelomic mass of mixed echogenicity containing fluid pockets. The ray was euthanatized and gross postmortem examination confirmed the presence of a fluid-filled coelomic mass in the region of the reproductive tract. The mass was identified histologically as a malignant round cell tumor of the ovary. Although immunohistochemistry for protein gene product 9.5 (PGP9.5), octamer-3/4 (OCT-3/4), and inhibin was attempted, antibodies that had been validated in mammalian species did not cross-react with stingray control tissues and did not label neoplastic cells. The final diagnosis was a presumptive dysgerminoma.