The low surface area, poor electrical conductivity, and rapid electron–hole recombination in bulk C₃N₄ limit its photocatalytic activity, which makes it challenging to improve the performance of bulk C₃N₄. Herein, an effective strategy is proposed to fabricate Co₃O₄/C₃N₄ heterojunctions (Co₃O₄ nanoparticles grown on C₃N₄ nanosheets), where bulk C₃N₄ is exfoliated to thin nanosheets. The bulk C₃N₄ precursor was synthesized with the hydrothermal treatment of melamine solution, and Co²⁺ ions were then inserted into the interlayer of the precursor through a vacuum-assisted intercalation process. Subsequently, the precursor was exfoliated to C₃N₄ nanosheets, and 15 nm Co₃O₄ nanoparticles were simultaneously formed using in situ thermal polycondensation. The Brunauer–Emmett–Teller (BET) specific surface area of the prepared heterojunction was 21 times higher than that of bulk C₃N₄, and thus more active sites were exposed on the surface of the heterostructure. Co₃O₄ nanoparticles contained oxygen vacancies, and the type-II transfer mechanism between these nanoparticles and C₃N₄ could be used to effectively separate photogenic carriers and improve the electron mobility. As expected, the heterostructure exhibited an excellent photocatalyzed degradation rate of 99.5% for methylene blue within 30 min (10 mg catalyst, wavelength >420 nm) under visible light irradiation, which was nearly three times higher than that of bulk C₃N₄. Electron paramagnetic resonance (EPR) analysis indicated that ˙O₂⁻ was the main reactive oxidizing species during the degradation process.