S-Methylmethionine (SMM) is a derivative of methionine with the chemical formula (CH3)2S+CH2CH2CH(NH3+)CO2−.

This cation is a naturally-occurring intermediate in many biosynthetic pathways owing to the sulfonium functional group.

The term was coined in 1950 by Garnett Cheney for uncharacterized anti-ulcerogenic[4] factors in raw cabbage juice that may help speed healing of peptic ulcers.

Speculated roles include methionine storage, use as a methyl donor, regulation of SAM.

SMM can be subsequently converted to dimethyl sulfide (DMS) during the malt kilning process, causing an undesirable flavor.