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Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α amino acid, meaning it contains an α amino group (which is in the protonated −NH3+ form under biological conditions), an α carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and a side chain isobutyl group, making it a non polar aliphatic amino acid. It is essential in humans, meaning the body cannot synthesize it: it must be obtained from the diet. Human dietary sources are foods that contain protein, such as meats, dairy products, soy products, and beans and other legumes. It is encoded by the codons UUA, UUG, CUU, CUC, CUA, and CUG. Leucine is named after the Greek word for "white": λευκός (leukós, "white"), after its common appearance as a white powder, a property it shares with many other amino acids. Like valine and isoleucine, leucine is a branched chain amino acid. The primary metabolic end products of leucine metabolism are acetyl CoA and acetoacetate; consequently, it is one of the two exclusively ketogenic amino acids, with lysine being the other. It is the most important ketogenic amino acid in humans. Leucine and β hydroxy β methylbutyric acid, a minor leucine metabolite, exhibit pharmacological activity in humans and have been demonstrated to promote protein biosynthesis via the phosphorylation of the mechanistic target of rapamycin (mTOR).

As a food additive, L leucine has E number E641 and is classified as a flavor enhancer.

The Food and Nutrition Board (FNB) of the U. S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002. For leucine, for adults 19 years and older, 42 mg/kg body weight/day.

As a dietary supplement, leucine has been found to slow the degradation of muscle tissue by increasing the synthesis of muscle proteins in aged rats. However, results of comparative studies are conflicted. Long term leucine supplementation does not increase muscle mass or strength in healthy elderly men. More studies are needed, preferably ones based on an objective, random sample of society. Factors such as lifestyle choices, age, gender, diet, exercise, etc. must be factored into the analyses to isolate the effects of supplemental leucine as a stand alone, or if taken with other branched chain amino acids (BCAAs). Until then, dietary supplemental leucine cannot be associated as the prime reason for muscular growth or optimal maintenance for the entire population. Both L leucine and D leucine protect mice against epileptic seizures. D leucine also terminates seizures in mice after the onset of seizure activity, at least as effectively as diazepam and without sedative effects. High blood levels of leucine are associated with insulin resistance in humans, mice, and rodents. This might be due to the effect of leucine to stimulate mTOR signaling. Dietary restriction of leucine and the other BCAAs can reverse diet induced obesity in wild type mice by increasing energy expenditure, and can restrict fat mass gain of hyperphagic rats.

Leucine toxicity, as seen in decompensated maple syrup urine disease, causes delirium and neurologic compromise, and can be life threatening. A high intake of leucine may cause or exacerbate symptoms of pellagra in people with low niacin status because it interferes with the conversion of L tryptophan to niacin. Leucine at a dose exceeding 500 mg/kg/d was observed with hyperammonemia. As such, unofficially, a tolerable upper intake level (UL) for leucine in healthy adult men can be suggested at 500 mg/kg/d or 35 g/d under acute dietary conditions.

Leucine is a dietary amino acid with the capacity to directly stimulate myofibrillar muscle protein synthesis. This effect of leucine results from its role as an activator of the mechanistic target of rapamycin (mTOR), leucine binding to leucyl tRNA synthetase, and possibly other mechanisms.

Leucine is a branched chain amino acid (BCAA) since it possesses an aliphatic side chain that is not linear. Racemic leucine had been subjected to circularly polarized synchrotron radiation to better understand the origin of biomolecular asymmetry. An enantiomeric enhancement of 2.6% had been induced, indicating a possible photochemical origin of biomolecules' homochirality.