Spironolactone (marketed under the trade names Aldactone, Novo-Spiroton, Spiractin, Spirotone, or Berlactone) is a diuretic and is used as an antiandrogen.

It is a synthetic 17-lactone drug which is a renal competitive aldosterone antagonist in a class of pharmaceuticals called potassium-sparing diuretics, used primarily to treat ascites in patients with liver disease, low-renin hypertension, hypokalemia, and Conn's syndrome as well as high blood pressure. On its own, spironolactone is only a weak diuretic, but it can be combined with other diuretics. About one person in one hundred with hypertension has elevated levels of aldosterone; in these persons the antihypertensive effect of spironolactone may exceed that of complex combined regimens of other antihypertensives. Due to its anti-androgen effect, it can also be used to treat hirsutism, and is a common component in hormone therapy for male-to-female transsexual and transgendered people. It is also used for treating hair loss and acne in women.