Electrochemical and, more recently, photoelectrochemical CO2 reductions have been widely investigated to convert atmospheric CO2 into other useful chemicals. However, understanding the mechanism and selectivity of materials capable of reducing CO2 remains a challenge. Using plasmonic dendritic electrodes of silver and cuprous oxide (Ag/Cu2O) and employing Raman spectroelectrochemistry detection, we observe the selective photoelectroreduction mechanism from CO2 to acetate as a value-added compound instead of the more commonly reported products like methanol, ethylene, or ethane. The selectivity, efficiency, and low overpotential (−0.4 V vs Ag/AgCl) in the CO2 reduction is favored by the basic microenvironment, the semiconductor properties of the Cu2O, and the accumulation of hot electrons from the localized surface plasmon resonance of the Ag nanostructure. Lateral surface interactions between adsorbed CO species are the key to the formation of acetate. The rate-determining step of the reaction is the single transfer of an electron from the electrode to the CO2 molecule to reduce it to the *CO2 – radical anion and subsequently form adsorbed CO, which is a key intermediate in the formation of the carbon–carbon bond during the reduction process.