A pheochromocytoma (also phaeochromocytoma, English spelling) is a tumor in the medulla of the adrenal glands (or, rarely, the ganglia of the sympathetic nervous system) which secretes excessive amounts of catecholamines, usually epinephrine and norepinephrine. The signs and symptoms of a pheochromocytoma are those of sympathetic nervous system hyperactivity: elevated heart rate and blood pressure and anxiety often resembling that of a panic attack. A pheochromocytoma can be deadly because of the severe blood pressure elevation it produces.

One diagnostic test for a pheochromocytoma is to administer clonidine (Catapres®), a centrally-acting alpha-2 agonist used to treat high blood pressure. Clonidine mimics catacholamines in the brain, causing it to reduce the activity of the sympathetic nerves controlling the adrenal medulla. A healthy adrenal medulla will respond to clonidine by reducing catacholamine production; the lack of a response is evidence of pheochromocytoma. Another test is for the clinician to press gently on the adrenal gland. A pheochromocytoma will often release a burst of catacholamines, with the associated signs and symptoms quickly following.

Pheochromocytomae occur most often during young-adult to mid-adult life. Less than 10% of pheochromocytomas are malignant (cancerous).

These tumors can form a pattern with other endocrine gland cancers which is labelled multiple endocrine neoplasia. Pheochromocytoma is listed within the groups MEN 2a and MEN 2b.