Aspartic acid (symbol Asp or D;salts known as aspartates), is an α-amino acid that is used in the biosynthesis of proteins.

Similar to all other amino acids it contains an amino group and a carboxylic acid. Its α-amino group is in the protonated –NH+ 3 form under physiological conditions, while its α-carboxylic acid group is deprotonated −COO− under physiological conditions. Aspartic acid has an acidic side chain (CH2COOH) which reacts with other amino acids, enzymes and proteins in the body.

Under physiological conditions (pH 7.4) in proteins the side chain usually occurs as the negatively charged aspartate form, −COO−.

It is a non-essential amino acid in humans, meaning the body can synthesize it as needed. It is encoded by all the codons GAU and GAC.