Trimethylamine (TMA) is an organic compound with the formula N(CH₃)₃. It is a trimethylated derivative of ammonia. TMA is widely used in industry. At higher concentrations it has an ammonia-like odor, and can cause necrosis of mucous membranes on contact. At lower concentrations, it has a "fishy" odor, the odor associated with rotting fish.

TMA is a colorless, hygroscopic, and flammable tertiary amine. It is a gas at room temperature but is usually sold as a 40% solution in water. It is also sold in pressurized gas cylinders.

TMA protonates to give the trimethylammonium cation. Trimethylamine is a good nucleophile, and this reactivity underpins most of its applications. Trimethylamine is a Lewis base that forms adducts with a variety of Lewis acids.

Trimethylamine is prepared by the reaction of ammonia and methanol employing a catalyst:

This reaction coproduces the other methylamines, dimethylamine (CH₃)₂NH and methylamine CH₃NH₂.

Trimethylammonium chloride has been prepared by a reaction of ammonium chloride and paraformaldehyde:

Trimethylamine is produced by several^[which?] routes in nature. Well studied are the degradation of choline and carnitine.

Trimethylamine is used in the synthesis of choline, tetramethylammonium hydroxide, plant growth regulators, herbicides, strongly basic anion exchange resins, dye leveling agents and a number of basic dyes. Gas sensors to test for fish freshness detect trimethylamine.