A 12-year-old, spayed female, Toy-poodle dog presented in progressive, pruritic carpal mass and mandibular papule. The skin lesions and enlarged superficial lymph nodes were surgically removed. According to histopathology and immunohistochemistry, the patient diagnosed as cutaneous epitheliotropic T-cell lymphoma with lymph node metastasis. Chemotherapy with L-asparaginase, lomustine and prednisolone was initiated. The patient deteriorated despite treatment and was admitted to the hospital. Blood tests revealed significant leukocytosis. Circulating large lymphocytes with convoluted and cerebriform nuclei seen in peripheral blood led to presumption of Sézary cells. Thus, considering skin, lymph nodes and blood involvement, Sézary syndrome was diagnosed.