Trimethylsulfoxonium (abbreviated TMSO) is a cation with a formula (CH3)3SO+ consisting of a sulfur atom attached to three methyl groups and one oxygen atom.

Refluxing dimethyl sulfoxide with methyl iodide can yield trimethylsulfoxonium iodide.

[1] Treated with sodium hydride, trimethylsulfoxonium forms dimethylsulfoxonium methylide.

[1] Trimethylsulfoxonium can polymerise to yield polyethylene.

[2][3] Copper, zinc and palladium ions in water react with trimethylsulfoxonium and sodium hydroxide to form sulfur ylide complexes.