Formyl methionine (fMet) is a modified amino acid that initiates protein synthesis in prokaryotes. It is derived from methionine by the addition of a formyl group to the amino group. In eukaryotes, methionine is the first amino acid added to the polypeptide chain.

The use of fMet in translation differs between prokaryotes and eukaryotes. In prokaryotes, fMet is rarely retained as the N-terminal amino acid. In roughly half of all proteins, the formyl group is removed by the enzyme deformylase, leaving methionine as the N-terminal amino acid. In some cases, methionine and possibly a few more amino acids are also removed. In eukaryotes, the whole methionine is usually removed from the polypeptide chain by methionine aminopeptidase (MAP), depending on the size of the residue adjacent to methionine.