The design of high-performance and earth-abundant electrocatalysts for electrochemical carbon dioxide (CO₂) reduction reactions (CO₂RRs) to form fuels and value-added chemicals offers an emergent solution for environmental and sustainable energy issues. Herein, we report a facile and one-step electrochemical fabrication of a series of hierarchical three-dimensional (3-D) copper selenide nanocubes (CuSe NCs) assembled as domain nanocube-, branched nanocube-, and dendrite nanocube-like surface morphologies on copper microelectrodes (CuMEs). Our results reveal that the peculiar morphology-controllable CuSe NCs are grown on a Cu microelectrode surface due to the preferential diffusion of Cu and Se ions and deposition as CuSe heterostructures. The as-developed CuSe NCs are employed as potential electrocatalysts for electrochemical CO₂RRs in both 1.0 M KHCO₃ and 0.1 M 1-butyl-3-methyl-imidazolium hexafluoro-phosphate ([Bmim]PF₆) in acetonitrile (MeCN) electrolytes. The as-developed CuSe NCs-B|CuMEs exhibited a less negative onset potential (∼0.5 V (RHE) in 1.0 M KHCO₃ and ∼1.1 V (Ag/AgCl) in 0.1 M [Bmim]PF₆/MeCN) and high cathodic current density (∼120.3 mA cm⁻² @ −1.3 V (RHE) in 1.0 M KHCO₃ and ∼23.4 mA cm⁻² @ 2.0 V (Ag/AgCl) in 0.1 M [Bmim]PF₆/MeCN). The CuSe NCs-B|CuMEs showed a low cathodic potential of ∼0.65 V vs. RHE (1.0 M KHCO₃) and ∼1.6 V vs. Ag/AgCl (0.1 M [Bmim]PF₆/MeCN) to attain ∼10 mA cm⁻², small polarization resistance, high mass activity, and long-term durability. The attained high electrocatalytic performance of the CuSe NC-based Cu microelectrodes is due to high crystallinity, hierarchical 3-D nanocube-like morphology, a large volume of electrochemical active sites, low polarization resistance, and integrated intrinsic catalytic activity.