A photocatalyst composed of Ag nanoparticles (NPs) supported on pseudo-hexagonal niobium pentoxide (TT-Nb₂O₅) was synthesized in this study. This catalyst exhibited an enhanced photocatalytic performance for the reduction of CO₂ by H₂O under UV–vis light irradiation. A CO yield of up to 66.5 μmol·g⁻¹·h⁻¹ with a selectivity of 92.1% were achieved, and also an excellent stability was observed across four cycles. Characterization studies show that the introduction of Ag NPs onto Nb₂O₅ was beneficial for the formation of oxygen vacancies (V_os) (which were further regenerated when exposed to UV light), resulting in the enhanced adsorption and activation of CO₂. The electron-rich Ag NPs were conducive to the formation of Ag-H bonds after the dissociation of H₂O to H⁺, which promoted the proton-electron coupling process for converting the CO₂ absorbed at V_os sites into CO. Moreover, the localized surface plasmon resonance (LSPR) of Ag NPs，which was induced by visible light, playing a role in accelerating the decomposition and desorption of *COOH into CO. This work provides new insight into reduction of CO₂ by H₂O under UV–vis light irradiation.