Conventional methods for synthesizing protein/peptide–polymer conjugates, as a means to improve the pharmacological properties of therapeutic biomolecules, typically have drawbacks including low yield, non‐trivial separation of conjugates from reactants, and lack of site‐ specificity, which results in heterogeneous products with significantly compromised bioactivity. To address these limitations, the use of sortase A from Staphylococcus aureus is demonstrated to site‐specifically attach an initiator solely at the C‐terminus of green fluorescent protein (GFP), followed by in situ growth of a stealth polymer, poly(oligo(ethylene glycol) methyl ether methacrylate) by atom transfer radical polymerization (ATRP). Sortase‐catalyzed initiator attachment proceeds with high specificity and near‐complete (≈95%) product conversion. Subsequent in situ ATRP in aqueous buffer produces 1:1 stoichiometric conjugates with >90% yield, low dispersity, and no denaturation of the protein. This approach introduces a simple and useful method for high yield synthesis of protein/peptide–polymer conjugates.
