S-Methylcysteine sulfoxide is an organosulfur compound with the formula CH3S(O)CH2CH(NH2)CO2H.

It contributes to the flavor of onions (Allium sp.).

[1][2] The compound is usually encountered as one diastereoisomer, the (R)- and (S)-configurations at the carbon and sulfur stereocenters, respectively.

The S-methyl part of the name refers to the location of the methyl group on sulfur.

Although odorless, S-methylcysteine sulfoxide is acted upon by alliinase and lachrymatory-factor synthase.