In this paper, we report for the first time the significant photocatalytic activity of Nb-based materials for CO2 reduction. Nb2O5 catalysts were prepared through a modified peroxide sol-gel method using different annealing temperatures, showing activity for CO2 photoreduction in all conditions. The activity and selectivity of the Nb2O5 samples were directly related to their surface acidity: high surface acidity prompted conversion of CO2 to CO, HCOOH, and CH3COOH; low surface acidity prompted conversion of CO2 to CH4. The results also indicated that CO is the main intermediate species of the CO2 photoreduction in all conditions. We have uncovered the role played by the surface acidity of Nb2O5 and the mechanism behind its performance for CO2 photoreduction.