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The synthesis of protein-protein and protein-peptide conjugates is an important capability for producing vaccines, immunotherapeutics, and targeted delivery agents. Herein we show that the enzyme tyrosinase is capable of oxidizing exposed tyrosine residues into o -quinones that react rapidly with cysteine residues on target proteins. This coupling reaction occurs under mild aerobic conditions and has the rare ability to join full-size proteins in under 2 h. The utility of the approach is demonstrated for the attachment of cationic peptides to enhance the cellular delivery of CRISPR-Cas9 20-fold and for the coupling of reporter proteins to a cancer-targeting antibody fragment without loss of its cell-specific binding ability. The broad applicability of this technique provides a new building block approach for the synthesis of protein chimeras., Competing Interests: The authors declare the following competing financial interest(s): J.A.D. is an Investigator of the Howard Hughes Medical Institute; executive director of the Innovative Genomics Institute at the University of California, Berkeley and the University of California, San Francisco; a co-founder of Editas Medicine, Intellia Therapeutics, and Caribou Biosciences; and a scientific adviser to Caribou, Intellia, eFFECTOR Therapeutics, and Driver. The Regents of the University of California have patents pending for CRISPR technologies and the use of tyrosinase in the manner described herein on which the authors are inventors., (Copyright © 2020 American Chemical Society.)