Homocysteic acid is the organosulfur compound with the formula . A white solid, it is sulfonic acid-containing non-proteinogenic amino acid. It is an analog of glutamic acid and is a potent NMDA receptor agonist. It is related to homocysteine, a by-product of methionine metabolism.
Homocysteic acid is prepared by the oxidation of homocystine with aqueous bromine.