Severe splenomegaly was found during routine examination of a clinically normal 7-yr-old male Asian small clawed otter. The spleen and three enlarged splenic lymph nodes were immediately removed. The spleen weighed 310 g (approximately 8% of body weight). The spleen and resected lymph nodes were diffusely infiltrated by coalescing sheets of neoplastic lymphocytes that occasionally surrounded remnants of preexisting lymphoid follicles. Immunohistochemical confirmation of B lymphocyte origin and microscopic pattern were consistent with primary splenic marginal zone lymphoma (MZL) with metastasis to the splenic lymph nodes. The otter received no additional treatment and survived for 16 mo following splenectomy. Necropsy confirmed metastasis to multiple abdominal and extra-abdominal lymph nodes, liver, and kidney, and renal failure related to glomerulosclerosis. The prolonged survival in this otter is typical for MZL, an indolent form of B-cell lymphosarcoma that spreads slowly to the abdominal and extra-abdominal lymph nodes.