In winter 1990, an adult male sea otter (Enhydra lutris) was found dead along the eastern shore of Prince William Sound, Alaska. Necropsy findings included an enlarged retained left testicle with a twisted spermatic cord, enlarged left sublumbar lymph node, emaciation, dental attrition, oral papules and ulcers, and luminal intestinal hemorrhage associated with numerous acanthocephalids. A malignant seminoma was present in the left testicle and left sublumbar lymph node. Additionally, herpesvirus like intranuclear inclusion bodies were found in oral, esophageal, and corneal epithelial cells. Virions consistent with a herpesvirus were found in esophageal epithelium. Dental attrition, severe intestinal acanthocephaliasis, the malignant seminoma, and emaciation were considered contributing factors in causing death. The herpesviral disease was probably secondary to the debilitation and stress. This is the first report of malignant seminoma with metastasis in a sea otter.