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Peptides typically have poor biostabilities, and natural sequences cannot easily be converted into drug‐like molecules without extensive medicinal chemistry. We have adapted mRNA display to drive the evolution of highly stable cyclic peptides while preserving target affinity. To do this, we incorporated an unnatural amino acid in an mRNA display library that was subjected to proteolysis prior to selection for function. The resulting “SUPR (scanning unnatural protease resistant) peptide” showed ≈500‐fold improvement in serum stability (t =160 h) and up to 3700‐fold improvement in protease resistance versus the parent sequence. We extended this approach by carrying out SUPR peptide selections against Her2‐positive cells in culture. The resulting SUPR4 peptide showed low‐nanomolar affinity toward Her2, excellent specificity, and selective tumor uptake in vivo. These results argue that this is a general …