Addison's disease, also known as primary adrenal insufficiency, is a rare long-term endocrine disorder characterized by inadequate production of the steroid hormones cortisol and aldosterone by the two outer layers of the cells of the adrenal glands (adrenal cortex), causing adrenal insufficiency. Symptoms generally come on slowly and insidiously and may include abdominal pain and gastrointestinal abnormalities, weakness, and weight loss. Darkening of the skin in certain areas may also occur. Under certain circumstances, an adrenal crisis may occur with low blood pressure, vomiting, lower back pain, and loss of consciousness. Mood changes may also occur. Rapid onset of symptoms indicates acute adrenal failure, which is a clinical emergency. An adrenal crisis can be triggered by stress, such as from an injury, surgery, or infection.

Addison's disease arises from problems with the adrenal gland such that not enough of the steroid hormone cortisol and possibly aldosterone are produced. It is an autoimmune disease which affects some genetically predisposed people in whom the body's own immune system has started to target the adrenal gland. While it can follow tuberculosis, in many adult cases it is unclear what has triggered onset of the disease. Causes can include certain medications, sepsis, and bleeding into both adrenal glands. Addison's disease is generally diagnosed by blood tests, urine tests, and medical imaging (adrenal crises can occur in all forms of adrenal insufficiency)

Treatment involves replacing the absent hormones. This involves taking a synthetic corticosteroid, such as hydrocortisone or fludrocortisone. These medications are usually taken by mouth. Lifelong, continuous steroid replacement therapy is required, with regular follow-up treatment and monitoring for other health problems. A high-salt diet may also be useful in some people. If symptoms worsen, an injection of corticosteroid is recommended (people need to carry a dose with them). Often, large amounts of intravenous fluids with the sugar dextrose are also required. With appropriate treatment, long-term outcomes are typically favorable, and most people are able to lead a reasonably normal life. Without treatment, an adrenal crisis can result in death.

Addison's disease affects about 9 to 14 per 100,000 people in the developed world. It occurs most frequently in middle-aged females. Secondary adrenal insufficiency is more prevalent. The disease is named after Thomas Addison, a graduate of the University of Edinburgh Medical School, who first described the condition in 1855 (the adjective "addisonian" is sometimes also used).