Nitrogen dioxide (NO2), a significant atmospheric pollutant primarily emitted from combustion processes, poses substantial environmental and health risks, underscoring the critical need for accurate detection methods. Conventional metal oxide semiconductor sensors often suffer from limitations such as high operating temperatures and insufficient sensitivity. This study addresses these challenges by introducing a synergistic approach that combines noble metal modification with light excitation, enabling high-performance NO2 detection at room temperature. Au/SnO2 composites with hierarchical nanosheet architectures were synthesized via hydrothermal and impregnation methods, as confirmed by SEM. Their sensing properties were systematically investigated under light excitation. Among the samples tested, the 2.5 wt% Au/SnO2 sensor demonstrated superior performance, exhibiting a remarkable response, a low detection limit, and excellent selectivity, long-term stability, and humidity tolerance at room temperature. First-principles calculations reveal that the enhanced sensor performance originates from a synergistic effect between Au nanoparticle decoration, which enhances NO2 adsorption and charge transfer, and the additional photogenerated carriers induced by light excitation. This study provides novel material design insights for developing high efficiency room temperature NO2 sensors.