Second malignancies are frequent complications in human patients with chronic lymphocytic leukemia (CLL). However, the clinical details and outcome of this phenomenon were unclear in their canine counterparts. Here, we report a dog with high-grade lymphoma concurrent with T-cell CLL. A 10-year-old male golden retriever presented with lymphadenopathies. The lymph nodes contained large-sized lymphocytes, raising suspicion of high-grade lymphoma. Meanwhile, small lymphocytic lymphocytosis in the peripheral blood was consistent with CLL. Interestingly, molecular biological analyses revealed that CLL cells were of the T-cell type, whereas lymphoma cells were of the B-cell type. Chemotherapy using the L-VCA short protocol was effective for 155 days, but the dog died on day 194 after diagnosis, despite rescue therapies.