An asymptomatic 14-year old, male black swan (Cygnus atratus) housed at a zoological institution was presented for routine preshipment examination. Hematologic findings indicated that the bird had a severe lymphocytic leukocytosis, consistent with chronic lymphocytic leukemia. Radiographs showed the presence of multiple soft tissue masses within the caudal coelomic cavity; ultrasound showed one mass to be an enlarged spleen, a cystic mass near the gonads, and a mass suspected to be associated with the ventriculus. Results of further antemortem diagnostics, including bone marrow aspiration, fine-needle aspirate cytology of the coelomic masses, and immunohistochemical staining confirmed T-cell leukemia with infiltration of the bone marrow and the spleen. The bird showed partial response to treatment with chlorambucil, lomustine, prednisone, l-asparaginase, and whole-body radiation, with neither evidence of adverse effects nor clinical signs of disease. Although the leukemia showed response, there was no evidence of remission at any point. The swan died 433 days after initial evaluation and initiation of therapy. Necropsy, histopathologic findings, and immunohistochemistry results confirmed extensive infiltration of multiple organs, including the liver, spleen, heart, lungs, and kidneys with neoplastic T-cell lymphocytes.