(also α-amino-β-sulfonylpropionic acid), HO₃SCH(NH₂)COOH; an amino acid containing sulfur. Cysteic acid is an intermediate in the metabolism of cysteine and cystine in animals. It participates in transamination reactions, and it is a precursor of taurine.

The oxidation of cysteine and cystine residues—components of peptides and proteins—yields cysteic acid. This oxidation reaction is used to determine the number and position of sulfhydryl groups (—SS) and disulfide groups (—S—S—) in various peptides and proteins. From this information, other information can be derived on the spatial structure of peptide chains and the arrangement of the chains in protein molecules.