Plasmonic metal nanoparticles (NPs) have emerged as promising visible light harvesters to facilitate solar-to-chemical energy conversion via the generation of hot electrons by non-radiative decay of plasmons. As one of the most promising renewable energy production methods for the future, electrocatalytic water splitting is an ideal chemical reaction in which plasmonic NPs can be utilized for direct solar-to-fuel conversion. Due to the rapid carrier recombination on plasmonic NPs, hybrid photocatalysts integrating metals and semiconductors are usually employed to separate the hot electrons and holes. However, an understanding of the catalytic mechanism, which is critical for rational design of plasmonic electrocatalysts, including the interfacial charge transfer pathway and real reactive sites, has been lacking. Herein, we report on the combination of plasmonic Au NPs and semiconductors (Ni and/or Co hydroxides) for plasmon-promoted electrocatalytic water splitting. By using surface-enhanced Raman spectroscopy (SERS), we find a strong spontaneous interfacial charge transfer between Au and NiCo layered double hydroxide (LDH), which facilitates both the oxygen and hydrogen evolution reactions. The real catalytic sites on the hybrid material are confirmed by selective blocking of the metal surface with a thiol molecular monolayer. It is found that the plasmon-promoted oxygen evolution occurs on the LDH semiconductor but surprisingly, the hydrogen evolution sites are mainly located on the Au NP surface. This work demonstrates the critical role of interfacial charge transfer in hot electron-driven water splitting and paves the way for rational design of high-performance plasmonic electrocatalysts.