Abstract: Platinum is one of the most efficient cocatalysts for photocatalytic reduction of carbon dioxide (CO₂) to methane (CH₄), but it still suffer from low CO₂ reduction rate and low selectivity of CH₄. In this study, Pt/TiO₂ catalysts with adjustable Pt particle size (0.55−1.80 nm) were prepared by atomic layer deposition (ALD) and used for photocatalytic reduction of CO₂ to CH₄. The CH₄ yield and selectivity of the Pt/TiO₂ catalysts showed a volcanic variation trend with the increase of Pt particle size. The 10Pt/TiO₂ with Pt particle size of 1.35 nm exhibit the highest methane yield (71.9 μmol/(g·h)). Especially, a high electron-based selectivity of 81.69% for CH₄ (product-based selectivity of 90.20%), and 100% for hydrocarbons (CH₄, C₂H₆ and C₃H₈) are achieved, no H₂ formation was detected. The CO-DRIFTS, XPS, CO₂-TPD, H₂O-TPD, and H₂-TPD characterizations suggest that the 10Pt/TiO₂ exhibited optimal CO₂ adsorption/activation capacity, suitable H₂O activation capacity, and higher hydrogen desorption temperature, making the generation rate of active hydrogen species from H₂O matches their consumption rate for CO₂ hydrogenation. This study opens an avenue for rationally designing highly efficient and selective photocatalysts for photocatalytic CO₂ reduction.