Trimethylsulfonium (systematically named trimethylsulfanium) is an organic cation with the chemical formula (CH3)3S+ (also written as C3H9S+).

X-ray crystallography reveals that the ion has trigonal pyramidal molecular geometry at sulfur atom, with C-S-C angles near 102° and C-S bond distance of 177 picometers.

Unless the counteranion is colored, all trimethylsulfonium salts are white or colorless.

Sulfonium compounds can be synthesised by treating a suitable alkyl halide with a thioether.

Glyphosate herbicide is often supplied as a trimethylsulfonium salt, referred to as trimesium.