Hybrid photocatalysts constructed with a mononuclear Ru(ii)-complex (RuP), silver nanoparticles, and carbon nitride nanosheets (NS-C3N4) photocatalyze CO2 reduction to selectively form formate under visible light. The structure of the nanoparticulate silver species, which worked as promoters for the reaction, was characterized by X-ray diffraction, UV-VIS diffuse reflectance spectroscopy, high-resolution transmission microscopy, and X-ray absorption fine-structure spectroscopy. The silver promoters were loaded on the surface of NS-C3N4 by an impregnation method from an aqueous solution containing AgNO3 or an in situ photodeposition method. Impregnation of NS-C3N4 with 2.0 wt% Ag followed by reduction with H2 at 473 K (further modified with RuP) resulted in the highest photocatalytic activity, giving a turnover number of 5700 (based on RuP), which was the greatest value among the formate-generating hybrid systems with a mononuclear complex. While the optimized photocatalyst contained highly dispersed Ag2O-like nanoclusters as the major silver species, experimental results suggested that highly dispersed Ag0 species are more important for enhancing CO2 reduction activity, that is, the obtained experimental results led us to conclude that there are two major factors affecting activity: one is the feature size of silver species (smaller is better), and the other is the oxidation state of silver (metallic is better).