3,4,5-Trimethoxyamphetamine (TMA, TMA-1, or 3,4,5-TMA), also known as α-methylmescaline (3C-mescaline or 3C-M) or mescalamphetamine, is a psychedelic drug of the phenethylamine and amphetamine families. It is one of the trimethoxyamphetamine (TMA) series of positional isomers. The drug is notable in being the amphetamine (i.e., α-methylated) analogue of mescaline (3,4,5-trimethoxyphenethylamine).

TMA is a serotonergic psychedelic and produces hallucinogenic effects. It is said to be active at doses of 100 to 250 mg and to have a duration of 6 to 8 hours. For comparison, mescaline is typically used at doses of 200 to 500 mg and is said to have a duration of 10 to 12 hours or longer. TMA's positional isomer 2,4,5-trimethoxyamphetamine (2,4,5-TMA or TMA-2) is much more potent than TMA, with a dose of 20 to 40 mg and a duration of 8 to 12 hours.

TMA is a low-potency serotonin 5-HT_2A receptor partial agonist, with an affinity (K_i) of >12,000 nM, an EC₅₀Tooltip half-maximal effective concentration of 1,700 nM, and an E_maxTooltip maximal efficacy of 40%. Conversely, it was inactive at the serotonin 5-HT_1A, 5-HT_2B and 5-HT_2C receptors and at several other receptors, at least at the assessed concentrations (up to 10,000 nM). It showed affinity for the mouse and rat trace amine-associated receptor 1 (TAAR1) (K_i = 1,800 nM and 3,200 nM, respectively), whereas it was inactive at the human TAAR1 (EC₅₀ > 10,000 nM).

TMA is also a very low-potency serotonin releasing agent (SRA), with an EC₅₀ value of 16,000 nM. In contrast, it is inactive as a releasing agent and reuptake inhibitor of dopamine and norepinephrine (EC₅₀ > 100,000 nM). Despite its apparent SRA activity in vitro, TMA did not increase brain serotonin or dopamine levels in rodents in vivo. TMA is similarly inactive as a monoamine oxidase inhibitor (MAOI), including of both monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) (IC₅₀Tooltip half-maximal inhibitory concentration > 200,000 nM).

The low potency of TMA as a serotonin 5-HT_2A receptor agonist is analogous to the case of mescaline, which is a well-known and widely used psychedelic but is likewise a very low-potency agonist of this receptor, showing an affinity (K_i) of 9,400 nM, an EC₅₀ of 10,000 nM, and an E_max of 56% in the same study. For comparison, DOM has shown an affinity (K_i) of 88 nM and an EC₅₀ of 4 to 24 nM.

TMA was first synthesized by Gordon Alles around 1937. He assessed it in both animal studies and self-experiments and documented its effects, but these were not reported until 1959. The drug was first described in the scientific literature in 1947 and its psychedelic effects were first described in 1955. TMA was studied at Edgewood Arsenal under the code name EA‐1319 in 1953 and 1954. The drug was further characterized by Alexander Shulgin and described in his book PiHKAL.

TMA is a Schedule I controlled substance in the United States.