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Lysine‐specific peptide and protein modification strategies are widely used to study charge‐related functions and applications. However, these strategies often result in the loss of the positive charge on lysine, significantly impacting the charge‐related properties of proteins. Herein, we report a strategy to preserve the positive charge and selectively convert amines in lysine side chains to amidines using nitriles and hydroxylamine under aqueous conditions. Various unprotected peptides and proteins were successfully modified with a high conversion rate. Moreover, the reactive amidine moiety and derived modification site enable subsequent secondary modifications. Notably, positive charges were retained during the modification. Therefore, positive charge‐related protein properties, such as liquid‐liquid phase separation behaviour of α‐synuclein, were not affected. This strategy was subsequently applied to a lysine rich protein to develop an amidine‐containing coacervate DNA complex with outstanding mechanical properties. Overall, our innovative strategy provides a new avenue to explore the characteristics of positively charged proteins. [ABSTRACT FROM AUTHOR]