Enzymes and ribosomes accurately form peptide bonds without protecting groups in vivo. Inspired by biological chemistries, we have been exploring a chemoselective reaction for peptide bond formation. In this manuscript, we describe amino thioacid coupling (ATC) that enables peptide bond formation between a peptide-aryl thioester and an amino thioacid. The results suggest that ATC proceeds through the formation of a thioanhydride intermediate afforded by a chemoselective thioacid-thioester exchange reaction.