In enzymology, a thiol S-methyltransferase (EC 2.1.1.9) is an enzyme that catalyzes the chemical reaction

S-adenosyl-L-methionine + a thiol S-adenosyl-L-homocysteine + a thioether

Thus, the two substrates of this enzyme are S-adenosyl methionine and thiol, whereas its two products are S-adenosylhomocysteine and thioether.

This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is S-adenosyl-L-methionine:thiol S-methyltransferase. Other names in common use include S-methyltransferase, thiol methyltransferase, and TMT. This enzyme participates in selenoamino acid metabolism.

• Borchardt RT, Cheng CF (1978). "Purification and characterization of rat liver microsomal thiol methyltransferase". Biochim. Biophys. Acta. 522 (2): 340–53. doi:10.1016/0005-2744(78)90068-2. PMID 623768.
• Bremer J; Greenberg DM (1961). "Enzymic methylation of foreign sulfhydryl compounds". Biochim. Biophys. Acta. 46 (2): 217–224. doi:10.1016/0006-3002(61)90746-6.
• Weisiger RA, Jakoby WB (1979). "Thiol S-methyltransferase from rat liver". Arch. Biochem. Biophys. 196 (2): 631–7. doi:10.1016/0003-9861(79)90317-5. PMID 485170.