Spectroelectrochemical analysis using surface plasmon scattering can provide a great deal of information on nucleation and growth mechanisms for nanoscale materials. In this study, we combined a transparent electrochemical thin-layer cell with dark-field spectroscopy to resolve the Cu deposition process on individual Ag nanocubes. During the deposition process, both electrochemical responses and plasmon scattering images were obtained, which were directly correlated with reduction kinetics and morphological evolution, respectively. By applying linear sweep potentials with variable sweep rates, three distinct morphologies, atop tetrapods, dendritic spheres, and multiple cubes, were uniformly generated. The atop tetrapods were formed at the bulk deposition potential, -0.34 V. At lower potential sweep rates, however, the maximum scattering peak wavelength and intensity, converted from the plasmon scattering images, significantly changed in the low potential region of -0.23 to -0.33 V. The plasmon scattering simulation and scanning electron microscopy images at the fixed potentials provided evidence that nucleation occurred on the Ag surface in this potential range. Such an underpotential deposition of Cu on Ag was hardly observed in bulk but was critical to induce the distinct morphology in the nanoscale. Based on these observations, the deposition mechanism was understood in detail: the atop tetrapods were generated by selective bulk deposition, whereas the dendritic spheres and multiple cubes were formed by underpotential nucleation and growth aided by a surfactant. This spectroelectrochemical tool with dark-field spectroscopy provides real-time and in situ analysis under the actual reaction conditions and therefore, would be versatile to analyze various heterogeneous reaction systems.