This report details 2 different presentations of lymphoma in captive Steller's eiders (Polysticta stelleri). A female Steller's eider, at least 10 years old, developed lameness and lethargy. A complete blood cell count (CBC) revealed a severely elevated total white blood cell (WBC) count with a lymphocytosis. A subsequent liver biopsy confirmed a diagnosis of lymphoma and lymphocytic leukemia. The female Steller's eider was euthanatized 11 days after presentation. On necropsy, neoplastic lymphoid infiltrates were present in multiple tissues, primarily in the skin and subcutis. The results of a CBC from an apparently healthy male Steller's eider, at least 14 years old, also indicated this bird had an increased WBC count with a lymphocytosis. Serum submitted for protein electrophoresis indicated that this bird had a monoclonal gammopathy. Lymphocytes isolated from the male Steller's eider had low levels of proliferation in response to mitogens. The bird survived nearly 2 years without treatment, and the WBC count continued to increase during this time. The male Steller's eider maintained good body condition and exhibited normal behavior throughout the course of the disease until a sudden decline just prior to death. Lymphoma was diagnosed based on histopathology results. Infiltrates of atypical lymphocytes were observed in the bone marrow, proventriculus, ventriculus, heart, and eyes.