Epinephrine or adrenaline, sometimes spelled "epinephrin" or "adrenalin" respectively, is a hormone. It is a catecholamine, a sympathomimetic monoamine derived from the amino acids phenylalanine and tyrosine. The Latin roots ad-+renes and the Greek roots epi-+nephros both literally mean "on/to the kidney" (referring to the adrenal gland, which sits atop the kidneys and secretes epinephrine).

Epinephrine is a "fight or flight" hormone which is released from the adrenal glands when danger threatens or in an emergency. When secreted into the bloodstream, it rapidly prepares the body for action in emergency situations. The hormone boosts the supply of oxygen and glucose to the brain and muscles, while suppressing other non-emergency bodily processes (digestion in particular).

Epinephrine plays a central role in the physiological response to threatening, exciting, or environmental stressor conditions such as high noise levels or bright light. It is secreted by the adrenal medulla. When released into the bloodstream, epinephrine binds to multiple receptors and has numerous effects throughout the body. It increases heart rate and stroke volume, dilates the pupils, and constricts arterioles in the skin and gut while dilating arterioles in leg muscles. It elevates the blood sugar level by increasing catalysis of glycogen to glucose in the liver, and at the same time begins the breakdown of lipids in fat cells. Like some other stress hormones, epinephrine has a suppressive effect on the immune system.