A substantial proportion of adrenal incidentalomas demonstrates subtle hormonal hypersecretion; however, adenomas that cosecrete aldosterone and cortisol are rare. We here report a case of an adrenal mass that was incidentally detected on a computed tomography scan in a 57-year-old man. The patient had a 10-year history of diabetes mellitus and a 5-year history of hypertension. Evaluation revealed hyperaldosteronemia with an elevated plasma aldosterone-to-renin ratio, hypokalemia, unsuppressed cortisol after dexamethasone administration, and elevated urinary free cortisol concentration. The appearance of the right adrenalectomy specimen indicated adrenal adenoma. Postoperatively, the blood glucose and blood pressure control improved and the urinary cortisol and aldosterone-to-renin ratio normalized. A complete endocrine evaluation in patients with incidentally discovered adrenal masses should be performed, even if the patient has a long-standing history of hypertension and diabetes, to avoid any postoperative adrenal crises.