We report the development of a new "click"-type reaction for polypeptide modification based on the chemoselective alkylation of thioether groups in methionine residues. The controlled synthesis of methionine polymers and their alkylation by a broad range of functional reagents to yield stable sulfonium derivatives are described. These "methionine click" functionalizations are compatible with deprotection of other functional groups, use an inexpensive, natural amino acid that is readily polymerized and requires no protecting groups, and allow the introduction of a diverse range of functionality and reactive groups onto polypeptides.