Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can synthesize enough for its use. It is also the most abundant excitatory neurotransmitter in the vertebrate nervous system. It serves as the precursor for the synthesis of the inhibitory gamma-aminobutyric acid (GABA) in GABA-ergic neurons.

Its molecular formula is C
₅H
₉NO
₄. Glutamic acid exists in three optically isomeric forms; the dextrorotatory L-form is usually obtained by hydrolysis of gluten or from the waste waters of beet-sugar manufacture or by fermentation. Its molecular structure could be idealized as HOOC−CH(NH
₂)−(CH
₂)₂−COOH, with two carboxyl groups −COOH and one amino group −NH
₂. However, in the solid state and mildly acidic water solutions, the molecule assumes an electrically neutral zwitterion structure ⁻OOC−CH(NH⁺
₃)−(CH
₂)₂−COOH. It is encoded by the codons GAA or GAG.

The acid can lose one proton from its second carboxyl group to form the conjugate base, the singly-negative anion glutamate ⁻OOC−CH(NH⁺
₃)−(CH
₂)₂−COO⁻. This form of the compound is prevalent in neutral solutions. The glutamate neurotransmitter plays the principal role in neural activation. This anion creates the savory umami flavor of foods and is found in glutamate flavorings such as MSG. In Europe it is classified as food additive E620. In highly alkaline solutions the doubly negative anion ⁻OOC−CH(NH
₂)−(CH
₂)₂−COO⁻ prevails. The radical corresponding to glutamate is called glutamyl.