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Synthetic bioconjugation at cysteine (Cys) residues in peptides and proteins has emerged as a powerful tool in chemistry. Soft nucleophilicity of the sulfur in Cys renders an exquisite chemoselectivity with which various functional groups can be placed onto this residue under benign conditions. While a variety of reactions have been successful at producing Cys-based bioconjugates, the majority of these feature sulfur-carbon bonds. We report Cys-borylation, wherein a benchtop stable Pt(II)-based organometallic reagent can be used to transfer a boron-rich cluster onto a sulfur moiety in unprotected peptides forging a boron-sulfur bond. Discovered Cysborylation proceeds at room temperature and is tolerant to a variety of functional groups present in complex polypeptides. The resultant bioconjugates show no additional toxicity compared to their Cys aryl-based congeners. Finally, we demonstrate how the developed Cys-borylation can enhance the proteolytic stability of the produced peptide bioconjugates while maintaining the binding affinity to a protein target.