Glycine N-methyltransferase (EC 2.1.1.20) is an enzyme that helps speed up a chemical reaction. The enzyme helps change the amino acid glycine into sarcosine. A methyl group is taken from a helper molecule called S-adenosyl methionine (SAM). After giving up its methyl group, SAM becomes S-adenosyl-L-homocysteine (SAH).

Glycine N-methyltransferase is part of a group of enzymes called methyl transferases. The official name for this group is S-adenosyl-L-methionine:glycine N-methyltransferase. Other names for this enzyme include glycine methyltransferase, S-adenosyl-L-methionine:glycine methyltransferase, and GNMT. This enzyme helps control how methyl groups are used in the liver and pancreas by changing the balance between SAM and SAH. It is stopped from working by a molecule called 5-methyltetrahydrofolate pentaglutamate. The product, sarcosine, is changed back into glycine by another enzyme called sarcosine dehydrogenase.