Trimethylarsine (abbreviated TMA or TMAs) is the chemical compound with the formula (CH₃)₃As, commonly abbreviated AsMe₃ or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building block to other organoarsenic compounds, and serves as a ligand in coordination chemistry. It has distinct "garlic"-like smell. Trimethylarsine had been discovered as early as 1854.

As predicted by VSEPR theory, AsMe₃ is a pyramidal molecule. The As-C distances average 1.519 Å, and the C-As-C angles are 91.83° This bond angle is a strong indication of a low, if any, hybridisation of the atomic orbitals, leaving the lone pair in the s-orbital buried in the inner regions of the arsenic atom, rather than pointing outward like the lone pair of the ammonia molecule.

Trimethylarsine can be prepared by treatment of arsenic oxide with trimethylaluminium:

Trimethylarsine is pyrophoric due to the exothermic nature of the following reaction, which initiates combustion:

Poisoning events due to a gas produced by certain microbes was assumed to be associated with the arsenic in paint. In 1893 the Italian physician Bartolomeo Gosio published his results on "Gosio gas" that was subsequently shown to contain trimethylarsine. Under wet conditions, the mold Scopulariopsis brevicaulis produces significant amounts of methyl arsines via methylation of arsenic-containing inorganic pigments, especially Paris green and Scheele's Green, which were once used in indoor wallpapers. Newer studies show that trimethylarsine has a low toxicity and could therefore not account for the death and the severe health problems observed in the 19th century.

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