A 10-year-old, female spayed mixed-breed dog with a history of vomiting and anorexia was examined. Abnormal findings of comlete blood count and serum chemistry included polycythemia, thrombocytopenia, hyper-globulinemia and hypoalbuminemia. Abdominal radiographs revealed severe unilateral renomegaly, and ultrasonography showed a left-sided renal mass. During the operation, left kidney was resected. Cytologial and histopathological examinations revealed neoplastic lymphoid proliferation with high mitotic figures in renal mass. Immunohistochemistry revealed tumor cells were CD3-positive and CD79${alpha}$-negative, consistent with T-cell lineage. The renal mass of this case was diagnosed as renal T cell lymphosarcoma.