A 10-year-old female sea otter exhibited convulsions, arrhythmia, hyperthermia, forced breathing and anorexia and died after a week. Histopathological examination revealed neoplastic proliferation of small round cells with scant cytoplasm and round or oval nuclei distributed mainly in the thalamus. The proliferation of neoplastic cells was observed in the cerebral parenchyma and perivascular areas. The neoplastic cells were immunopositive for CD3, but not CD20. No neoplastic proliferation of T-cells was found in other organs. Taken together, we diagnosed this case as a primary cerebral T-cell lymphoma. To our knowledge, this is the first case of primary cerebral T-cell lymphoma in a sea otter.