An 11-year-old male mixed breed dog diagnosed with Cushing's disease and diabetes mellitus was treated by hypophysectomy. After surgery, the hypercortisolemia disappeared and the diabetes status improved. The insulin requirement to control hyperglycemia gradually decreased. At 12 weeks after surgery, there was no requirement for insulin and we suspected the diabetes was completely resolved. In the present case, diabetes mellitus seems to be secondary to Cushing's disease. In conclusion, this mixed breed dog with coexisting Cushing's disease and diabetes mellitus is the first case showing the effectiveness of hypophysectomy to treat diabetes mellitus secondary to Cushing's disease in dogs.