Electronic structure intrinsically controls the light absorbance, redox potential, charge-carrier mobility, and consequently, photoreactivity of semiconductor photocatalysts. The conventional approach of modifying the electronic structure of a semiconductor photocatalyst for a wider absorption range by anion doping operates at the cost of reduced redox potentials and/or charge-carrier mobility, so that its photoreactivity is usually limited and some important reactions may not occur at all. Here, we report sulfur-doped graphitic C₃N₄ (C₃N_4-xS_x) with a unique electronic structure that displays an increased valence bandwidth in combination with an elevated conduction band minimum and a slightly reduced absorbance. The C₃N_4-xS_x shows a photoreactivity of H₂ evolution 7.2 and 8.0 times higher than C ₃N₄ under λ > 300 and 420 nm, respectively. More strikingly, the complete oxidation process of phenol under λ > 400 nm can occur for sulfur-doped C₃N₄, which is impossible for C₃N₄ even under λ > 300 nm. The homogeneous substitution of sulfur for lattice nitrogen and a concomitant quantum confinement effect are identified as the cause of this unique electronic structure and, consequently, the excellent photoreactivity of C ₃N_4-xS_x. The results acquired may shed light on general doping strategies for designing potentially efficient photocatalysts.