Display omitted •S-scheme Cu@CN/TiOx heterojunction was engineered.•Efficient charge separation in Cu@CN/TiOx leads to 99.5% NO3– removal efficiency.•Advanced characterizations and theoretical validate S-scheme heterojunction design.•A novel sequential NO3– to NO2– to N2 pathway was discussed.•The tandem catalyst drives 95.5% N2 selectivity in PCDN. The persistent challenge of nitrate (NO3–) pollution in water systems, profoundly impacting ecosystems and human health, demands effective remediation strategies. Here, we present an innovative S-scheme Cu@CN/TiOx heterojunction for high-efficiency photocatalytic denitrification (PCDN). Evaluation of its performance demonstrates exceptional NO3– removal efficiency (99.5%) and remarkable N2 selectivity (95.5%), with 13.7 and 4.1 times higher rate constants of NO3– removal than the individual Cu@CN and TiOx components. Notably, this catalyst exhibits impressive stability across multiple cycles, maintaining consistent high conversion rates and selectivity. Mechanistic insights unveiled a tandem reaction pathway where Cu@CN selectively reduced NO3– to NO2–, followed by the TiOx-mediated CO2•– generation, enabling highly selective NO2– to N2 conversion. This unique catalytic system, featuring spatially separated redox-active sites, presents a promising avenue for efficient, selective, and stable denitrification processes, offering significant potential in addressing water pollution challenges and advancing tandem catalytic systems.