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Bioorthogonal late‐stage diversification of structurally complex peptides has enormous potential for drug discovery and molecular imaging. In recent years, transition‐metal‐catalyzed C−H activation has emerged as an increasingly viable tool for peptide modification. Despite major accomplishments, these strategies largely rely on expensive palladium catalysts. We herein report an unprecedented cobalt(III)‐catalyzed peptide C−H activation, which enables the direct C−H functionalization of structurally complex peptides, and sets the stage for a multicatalytic C−H activation/alkene metathesis/hydrogenation strategy for the assembly of novel cyclic peptides.