Although it is well-known that the size can influence the surface plasmon resonance property of coinage metals and the electronic state of the Mott–Schottky junction formed at the metal/semiconductor interface, insights into how the size can be exploited to optimize the photocatalytic activity and selectivity of metal/semiconductor composites are lacking. Here we utilize operando SERS spectroscopy to identify the size effect on the electron-transfer dynamics and the direction at the Au/TiO2 interface. This effect was characterized by the photocatalytic reduction sites of p-nitrothiophenol, which were self-tracked with the SERS spectra from Au nanoparticle and inverse-opal structured TiO2, respectively. The size-dependent unidirectional/bidirectional transfer of photoinduced electrons at the Au/TiO2 interface was revealed by operando SERS spectroscopy, which enables the rational tuning of the reduction selectivity.