Interfacial charge transfer critically governs photocatalytic efficiency, yet quantitative correlation between surface electronic modulation and redox activity remains limited. Herein, a surface-hydroxylated sulfur-embedded g-C3N4 anchored on ultrathin BiOCl nanosheets (SCB) was constructed via a sonication-assisted hydrothermal route to investigate interfacial interaction and photocatalytic behaviour. The optimized SCB exhibited a pseudo-first-order rate constant of 0.0302 min−1 and an apparent photonic efficiency of 2.36 × 10−10 mol J−1 under visible irradiation, retaining > 70% activity after ten cycles. UV–DRS and XPS valence-band analysis revealed broadened visible absorption and band modulation, while steady-state photoluminescence showed 52% quenching without spectral shift, suggesting reduced carrier recombination. Binding-energy shifts (0.1–0.3 eV) in Bi 4f and C/N core levels further evidenced interfacial electron redistribution. Electron paramagnetic resonance (g = 2.029) and HRMS analysis indicated a superoxide-dominant oxidation pathway without accumulation of persistent intermediates. The enhanced activity is attributed to sulfur-induced band alignment and hydroxyl-mediated interfacial coupling, which facilitate directional charge migration and reactive oxygen species generation. ICP–MS confirmed negligible Bi leaching (<0.5 ppb), and supplementary cradle-to-gate assessment indicated modest material and energy intensities. These results highlight the potential of heteroatom-induced surface electronic modulation to stabilize heterojunctions for efficient visible-light photocatalysis.