Single-atom nickel catalysts hold great promise for photocatalytic water splitting due to their plentiful active sites and cost-effectiveness. Herein, we adopt a reactive-group guided strategy to prepare atomically dispersed nickel catalysts on red phosphorus. The hydrothermal treatment of red phosphorus leads to the formation of P−H and P−OH groups, which behave as the reactive functionalities to generate the dual structure of single-atom P−Ni and P−O−Ni catalytic sites. The produced single-atom sites provide two different functions: P−Ni for water reduction and P−O−Ni for water oxidation. Benefitting from this specific Janus structure, Ni-red phosphorus shows an elevated hydrogen evolution rate compared to Ni nanoparticle-modified red phosphorus under visible-light irradiation. The hydrogen evolution rate was additionally enhanced with increased reaction temperature, reaching 91.51 μmol h−1 at 70 °C, corresponding to an apparent quantum efficiency of 8.9 % at 420 nm excitation wavelength.