A mild and chemoselective method for the thioacylation of amines using alpha-keto acids and elemental sulfur has been developed. The key to the success of this transformation is the nucleophilic activation of elemental sulfur by thiols such as 1-dodecanethiol. A variety of functional groups, including unprotected hydroxyl, carboxyl, amide, sulfide, and tertiary amine moieties, are tolerated under the applied reaction conditions. To demonstrate the advantages of this method compared with conventional O-S exchange reactions using Lawesson's reagent or P2S5, thioamide moieties were introduced site-specifically into biologically active compounds.