In the last 20 years, extensive research has been reported on the use of plasmonic nanoparticles as a potential photocatalyst. However, the low conversion efficiency has still remained a major concern. Herein, we present a new photocatalytic reaction system based on Au nanoclusters (Au NCs) to enhance the conversion efficiency. Negatively charged Au NCs electrostatically interact with positively charged metal ions and form highly aggregated nanocrystals, which can efficiently capture a chemical substance in the reaction mixture. In such a reaction system, the distance between the electron donor and acceptor can be shortened, resulting in an efficient electron transfer process. We examined the electron transfer behavior in a nanocavity system via resazurin photoreduction and compared the reaction rate with that of a colloidal system, which is a commonly used reaction system. Evidently, the nanocavity system facilitated an enhanced reaction rate compared to that of the colloidal system. Furthermore, this nanocavity reaction system permitted multistep photoreactions and multi-electron transfer processes.