In enzymology, a homocysteine S-methyltransferase (EC 2.1.1.10) is an enzyme that catalyzes the chemical reaction

S-methylmethionine + L-homocysteine ${\displaystyle \rightleftharpoons }$ 2 L-methionine

Thus, the two substrates of this enzyme are S-methylmethionine and L-homocysteine, and it produces 2 molecules of L-methionine.

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:L-homocysteine S-methyltransferase. This enzyme participates in methionine metabolism.

Other names of this enzyme in common use include S-adenosylmethionine homocysteine transmethylase, S-methylmethionine homocysteine transmethylase, adenosylmethionine transmethylase, methylmethionine:homocysteine methyltransferase, adenosylmethionine:homocysteine methyltransferase, homocysteine methylase, homocysteine methyltransferase, homocysteine transmethylase, L-homocysteine S-methyltransferase, S-adenosyl-L-methionine:L-homocysteine methyltransferase, S-adenosylmethionine-homocysteine transmethylase, and S-adenosylmethionine:homocysteine methyltransferase.