Proximity labeling has been used to study protein-protein interactions and can also be used as a protein labeling tool. We developed a novel 14-amino acid peptide substrate for the proximity-labeling enzyme PafA. The N terminus of the peptide can be modified with biotin or fluorophores, which allows various chemical moieties to be ligated to the target protein. We used PafA-mediated peptide labeling to label antibodies with a biotin tag without affecting the antigen binding capacity of the antibody. Similar strategies can be used for other types of protein labeling in the future.