Plasmonic metal nanoparticles (NPs) are promising catalysts in photocatalytic reactions. Understanding the exact role of sites where two particles are approaching (hot spots) is important to achieve higher efficiency of photocatalysis, and promote the development of advanced plasmon-driven photocatalytic systems. Surface-enhanced Raman spectroscopy was employed to probe photocatalytic coupling reactions occurring at individual plasmonic nanojunctions that trap light to nanoscale while serving as nanoreactors. Compared with nanocavities fabricated using the small Ag NPs (70 nm or 82 nm), the 102 nm Ag NP-molecule-Au thin film nanojunction demonstrated enhanced reaction kinetics and catalytic efficiency. On the basis of the experimental results and theoretical modeling, it was concluded that the photochemical reaction dynamics and yields showed direct correlation with the local electric field enhancement at the nanojunction hot spot. The largely enhanced electric field generates increased hot plasmonic electrons, promoting chemical transformations of the adsorbed molecules.