Norepinephrine, also called Noradrenaline (BAN), or 4,5-β-trihydroxy Phenethylamine is a catecholamine with multiple roles including those as a hormone and a neurotransmitter. It is the hormone and neurotransmitter most responsible for vigilant concentration in contrast to the chemically similar hormone, dopamine, which is most responsible for cognitive alertness.

Medically it is used in those with severe hypotension. It does this by increasing vascular tone (tension of vascular smooth muscle) through α-adrenergic receptor activation.

Areas of the body that produce or are affected by norepinephrine are described as noradrenergic. The terms noradrenaline (from the Latin) and norepinephrine (from the Greek) are interchangeable, with noradrenaline being the common name in most parts of the world. However the U.S. National Library of Medicine has promoted norepinephrine as the favored name. It was discovered by Ulf von Euler in 1946.

One of the most important functions of norepinephrine is its role as the neurotransmitter released from the sympathetic neurons to affect the heart. An increase in norepinephrine from the sympathetic nervous system increases the rate of contractions in the heart. As a stress hormone, norepinephrine affects parts of the brain, such as the amygdala, where attention and responses are controlled. Norepinephrine also underlies the fight-or-flight response, along with epinephrine, directly increasing heart rate, triggering the release of glucose from energy stores, and increasing blood flow to skeletal muscle.

Norepinephrine is synthesized from dopamine by dopamine β-hydroxylase in the secretory granules of the medullary chromaffin cells. It is released from the adrenal medulla into the blood as a hormone, and is also a neurotransmitter in the central nervous system and sympathetic nervous system, where it is released from noradrenergic neurons in the locus coeruleus. The actions of norepinephrine are carried out via the binding to adrenergic receptors.