Plasmon‐mediated electrocatalysis that rests on the ability of coupling localized surface plasmon resonance (LSPR) and electrochemical activation, emerges as an intriguing and booming area. However, its development seriously suffers from the entanglement between the photoelectronic and photothermal effects induced by the decay of plasmons, especially under the influence of applied potential. Herein, using LSPR‐mediated CO2 reduction on Ag electrocatalyst as a model system, we quantitatively uncover the dominant photoelectronic effect on CO2 reduction reaction over a wide potential window, in contrast to the leading photothermal effect on H2 evolution reaction at relatively negative potentials. The excitation of LSPR selectively enhances the CO faradaic efficiency (17‐fold at −0.6 VRHE) and partial current density (100‐fold at −0.6 VRHE), suppressing the undesired H2 faradaic efficiency. Furthermore, in situ attenuated total reflection‐surface enhanced infrared absorption spectroscopy (ATR‐SEIRAS) reveals a plasmon‐promoted formation of the bridge‐bonded CO on Ag surface via a carbonyl‐containing C1 intermediate. The present work demonstrates a deep mechanistic understanding of selective regulation of interfacial reactions by coupling plasmons and electrochemistry. This work quantitatively disentangles the photoelectronic (PE) and photothermal (PT) effects in plasmon‐mediated electrocatalysis using CO2RR on Ag NPs electrode as the model system. The PE effect prevails over a wide potential window for the CO2RR to CO, while the PT effect dominates the H2 evolution reaction at high overpotentials. It is revealed spectroscopically that the carbonyl‐containing C1 intermediate applies to the plasmon‐promoted CO2RR to CO process.