We report here a series of nontoxic and stable bismuth-based perovskite nanocrystals (PeNCs) with applications for photocatalytic reduction of carbon dioxide to methane and carbon monoxide. Three bismuth-based PeNCs of general chemical formulas A3Bi2I9, in which cation A+ = Rb+ or Cs+ or CH3NH3 + (MA+), were synthesized with a novel ultrasonication top-down method. PeNC of Cs3Bi2I9 had the best photocatalytic activity for the reduction of CO2 at the gas-solid interface with formation yields 14.9 μmol g-1 of methane and 77.6 μmol g-1 of CO, representing a much more effective catalyst than TiO2 (P25) under the same experimental conditions. The products of the photocatalytic reactions were analyzed using a gas chromatograph coupled with a mass spectrometer. According to electron paramagnetic resonance and diffuse-reflectance infrared spectra, we propose a reaction mechanism for photoreduction of CO2 via Bi-based PeNC photocatalysts to form CO, CH4, and other possible side products.