Using renewable solar energy to CO₂ conversion to chemical fuels has been widely perceived as an alternative solution for simultaneously tackling the energy crisis and the greenhouse effect. Herein, the selectivity of the photothermocataltytic CO₂ reduction with H₂O vapor by single‐atom dispersed‐Ru loaded CdS is regulated. Under full solar spectrum irradiation, CO as the main product with 97.2% selectivity is observed on bare CdS, whereas the intentional introduction of optimized amount of Ru metal enables CO₂ reduction to CH₄ with 97.6% selectivity. A series of experiments and characterizations reveal that the single‐atom dispersed Ru serves as adsorption and active sites of CO₂ molecules as well as electron traps, resulting in great improvement in CO₂ uptake capability of the catalyst and the effective suppression in the recombination of photogenerated carriers. Moreover, the thermal effect synchronously induced by light has a positive influence on photocatalysis, which can prominently enhance the catalytic activity without altering the product selectivity.