We propose a universal dimer-doping strategy to improve the photocatalytic water splitting activity of Ru₁/TiO₂ single-atom catalysts, in which the N atom is simultaneously doped. First-principles calculations show that the N dopants promote the substitution of Ti with Ru on the anatase TiO₂(101) surface by chemical bonding between Ru and N, and increase the stability of the system. Isolated Ru₁ atoms act as active sites for the reduction of protons, and Ru₁–N₁/TiO₂ has a hydrogen evolution activity comparable to that of Pd. The impurity bands within the band gap lead to a significant red-shift of the absorption edge towards the visible region, improving the photoabsorption and photocatalytic performance of TiO₂ under sunlight. The reason is the charge compensation effect localized to the doped Ru–N dimers. We expect that this generic scheme that simultaneously realizes band-structure tailoring and reaction control also applies to other single-atom loaded oxide-based photocatalytic systems.