AbstractDeveloping efficient homojunctions on g‐C₃N₄ promises metal‐free photocatalysis to realize truly sustainable artificial photosynthesis. However, current designs are limited by hindered charge separation due to inevitable grain boundaries and random formation of ineffective homojunctions embedded within the photocatalyst. Here, efficient photocatalysis is driven by introducing effective surface homojunctions on chemically and structurally identical g‐C₃N₄ through leveraging its size‐dependent electronic properties. Using a top‐down approach, the surface layer of bulk g‐C₃N₄ is partially exfoliated to create sheet‐like g‐C₃N₄ nanostructures on the bulk material. This hierarchical design establishes a subtle band energy offset between the macroscopic and nanoscopic g‐C₃N₄, generating homojunctions while maintaining the chemical and structural integrities of the original g‐C₃N₄. The optimized g‐C₃N₄ homojunction demonstrates superior photocatalytic degradation of antibiotic pollutants at >96% efficiency in 2 h, even in different real water samples. It achieves reaction kinetics (≈0.041 min⁻¹) up to fourfold better than standalone materials and their physical mixture. Mechanistic studies highlight the importance of the unique design in boosting photocatalysis by effectively promoting interfacial photocarrier manipulation and utilization directly at the point‐of‐catalysis, without needing co‐catalysts or sacrificial agents. This work presents enormous opportunities for developing advanced and green photocatalytic platforms for sustainable light‐driven environmental, energy, and chemical applications.