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Photothermal catalysis is one of the most promising green catalytic technologies, while distinguishing the effects of hot electrons and local heating remains challenging. Herein, we reported that the actual reaction temperature of photothermal ammonia synthesis over carbon‐supported Ru catalyst can be measured based on Le Chatelier′s principle, enabling the hot‐electron contribution to be quantified. By excluding local heating effects, we established that the activation energy via photothermal catalysis was much lower than that of thermocatalysis (54.9 vs. 126.0 kJ mol−1), stemming from hot‐electron injection lowering the energy barriers for both N2 dissociation and intermediates hydrogenation. Furthermore, hot‐electron injection acted to suppress carbon support methanation, giving the catalyst outstanding operational stability over 1000 h. This work provides new insights into the hot‐electron effects in ammonia synthesis, guiding the design of high‐performance photothermal catalysts. [ABSTRACT FROM AUTHOR]