Currently, plasmonic nanoparticles (PNPs) are considered highly efficient enhancers of catalytic processes. Herein, we report a concept where plasmonic Ag0@SiO2 nanoparticles can reversibly switch-off an oxidation catalytic process under light-excitation. The catalytic process recommences when illumination is stopped. The catalytic system under study is a well-characterized molecular LMnII catalyst that performs alkene oxidation, with H2O2 as the oxidant. Three types of plasmonic core–shell Ag0@SiO2 nanoparticles, with a SiO2 shell of varying thickness (0.1–5 nm), were utilized in this study. Using electron paramagnetic resonance spectroscopy, we have identified the reversible inhibition of the transient LMnIV═O intermediate formation, to be the key-step of the photoinduced pause of the catalytic process by the Ag0@SiO2 PNPs. Surface-enhanced Raman spectroscopy (SERS) and redox potential data show that the plasmonic Ag0@SiO2 NPs exert a moderate SERS effect on the LMnII catalyst, and a considerable lowering of the solution redox potential Eh. Our data show that near-field generation is not the sole origin of inhibition of LMnIV═O formation, while plasmonic heating was insignificant. We suggest that the generation of hot electrons by the Ag0@SiO2 PNPs is implicated, along with near-field generation, in the reversible switch-off of the catalytic process.