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Abstract Antibiotic‐resistant bacterial strains are an ever‐present hurdle for human health. A route to overcoming this threat is the development of effective antimicrobial agents based on carbon‐supported nanocomposites. In this study, carbon dots (CD) are synthesized by a facile hydrothermal treatment of ethylenediaminetetraacetic acid and melamine and further functionalized with nickel hydroxide colloids. Whereas CD alone exhibits virtually no antimicrobial activity under photoirradiation at 365 nm againstEscherichia coliin comparison to the blank control, the performance is markedly enhanced with the Ni(OH)₂‐CD nanocomposites, with the lag time prolonged from 7 to 15 h and growth rate reduced by ca. 15%. This is ascribed to the Ni(OH)₂colloids that facilitate the separation of photogenerated electron‐hole pairs and ensuing production of superoxide radicals, as confirmed by photoluminescence and electron paramagnetic resonance measurements, which induce oxidative stress and damage to the bacterial cell membranes, thereby leading to effective bactericidal activity. Consistent results are obtained in live/dead assays. Results from this work highlight the unique potential of carbon‐based composites in the development of next‐generation antimicrobial agents.