Understanding the activation and transfer of hydrogen species at interfacial sites is essential for catalytic hydrogenation, yet still remains a significant challenge. Herein, the spillover of hydrogen species and its role in tuning the activity and selectivity in catalytic hydrogenation have been investigated  in situ  using surface‐enhanced Raman spectroscopy (SERS) with 10 nm spatial resolution through the precise fabrication of Au/TiO  2  /Pt sandwich nanostructures.  In situ  SERS study reveals that hydrogen species can efficiently spillover at Pt‐TiO  2  ‐Au interfaces, and the ultimate spillover distance on TiO  2  is ~50 nm. Combining kinetic isotope experiment and density functional theory calculation, we find that the hydrogen spillover proceeds via the water‐assisted cleavage and formation of surface hydrogen‐oxygen bond. More importantly, the selectivity in the hydrogenation of the nitro or isocyanide group is manipulated by controlling the hydrogen spillover. This work provides molecular insights to deepen the understanding of hydrogen activation and boosts the design of active and selective catalysts for hydrogenation.