Trimethylsulfoxonium iodide is an organosulfur compound with the chemical formula . It is a sulfoxonium salt derived from dimethylsulfoxide. It is iodide salt of a common sulfoxonium cation. This compound, a colorless solid, is commercially available. It may be prepared by the alkylation of dimethyl sulfoxide with iodomethane:
The trimethylsulfoxonium ion features a tetrahedral molecular geometry at sulfur center. The ion has idealized C<sub>3v</sub> symmetry. It is isoelectronic with trimethylphosphine oxide.
Trimethylsulfoxonium iodide is used to generate dimethyloxosulfonium methylide by reaction with sodium hydride. The latter compound is used to prepare epoxides from ketones and aldehydes.