A stranded adult female northern elephant seal (Mirounga angustirostris) was admitted to a rehabilitation hospital grossly underweight and lethargic in April 2001. The animal was icteric, had severe bilirubinemia, and elevated serum gamma glutamyl transferase concentrations. Laparoscopy under anesthesia revealed multiple masses up to 3 cm diameter throughout the liver and spleen and the animal was euthanized. Abnormal gross postmortem findings included green serous fluid in the abdominal cavity and 0.5 to 3 cm diameter yellow nodules scattered throughout the liver, spleen, and lymph nodes. Histopathology revealed a biliary adenocarcinoma with metastases to the liver, spleen, lymph nodes, adrenal gland, and pancreas. This is believed to be the first reported case of neoplasia in a northern elephant seal.