Arginine is the amino acid with the formula (H₂N)(HN)CN(H)(CH₂)₃CH(NH₂)CO₂H. The molecule features a guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO₂⁻) and both the amino and guanidino groups are protonated, resulting in a cation. Only the l-arginine (symbol Arg or R) enantiomer is found naturally. Arg residues are common components of proteins. It is encoded by the codons CGU, CGC, CGA, CGG, AGA, and AGG. The guanidine group in arginine is the precursor for the biosynthesis of nitric oxide. Like all amino acids, it is a white, water-soluble solid.

The one-letter symbol R was assigned to arginine for its phonetic similarity.

Arginine was first isolated in 1886 from yellow lupin seedlings by the German chemist Ernst Schulze and his assistant Ernst Steiger. He named it from the Greek árgyros (ἄργυρος) meaning "silver" due to the silver-white appearance of arginine nitrate crystals. In 1897, Schulze and Ernst Winterstein (1865–1949) determined the structure of arginine. Schulze and Winterstein synthesized arginine from ornithine and cyanamide in 1899, but some doubts about arginine's structure lingered until Sørensen's synthesis of 1910.

It is traditionally obtained by hydrolysis of various sources of protein, such as gelatin. It is obtained commercially by fermentation. In this way, 25-35 g/liter can be produced, using glucose as a carbon source.

Arginine is classified as a semiessential or conditionally essential amino acid, depending on the developmental stage and health status of the individual. Preterm infants are unable to synthesize arginine internally, making the amino acid nutritionally essential for them. Most healthy people do not need to supplement with arginine because it is a component of all protein-containing foods and can be synthesized in the body from glutamine via citrulline. Additional, dietary arginine is necessary for otherwise healthy individuals temporarily under physiological stress, for example during recovery from burns, injury or sepsis, or if either of the major sites of arginine biosynthesis, the small intestine and kidneys, have reduced function, because the small bowel does the first step of the synthesizing process and the kidneys do the second.

Arginine is an essential amino acid for birds, as they do not have a urea cycle. For some carnivores, for example cats, dogs and ferrets, arginine is essential, because after a meal, their highly efficient protein catabolism produces large quantities of ammonia which need to be processed through the urea cycle, and if not enough arginine is present, the resulting ammonia toxicity can be lethal. This is not a problem in practice, because meat contains sufficient arginine to avoid this situation.

Animal sources of arginine include meat, dairy products, and eggs, and plant sources include seeds of all types, for example grains, beans, and nuts.

Arginine is synthesized from citrulline in the urea cycle by the sequential action of the cytosolic enzymes argininosuccinate synthetase and argininosuccinate lyase. This is an energetically costly process, because for each molecule of argininosuccinate that is synthesized, one molecule of adenosine triphosphate (ATP) is hydrolyzed to adenosine monophosphate (AMP), consuming two ATP equivalents.^{[citation needed]}