Targeting tryptophan is a promising strategy to achieve high levels of selectivity for peptide or protein modification. A chemoselective peptide modification method via photocatalytic tryptophan β-position conjugation has been discovered. This transformation has good substrate scope for both peptide and Michael acceptor, and has good chemoselectivity versus other amino acid residues. The endogenous peptides, glucagon and GLP-1 amide, were both successfully conjugated at the tryptophan β-position. Insulin was studied as a nontryptophan control molecule, resulting in exclusive B-chain C-terminal-selective decarboxylative conjugation. This transformation provides a novel approach toward peptide modification to support the discovery of new therapeutic peptides, protein labeling and bioconjugation.