Herein, we demonstrate a three-dimensional (3D) nanoporous (NP) structure, resulting from electrochemical alloying/dealloying of polycrystalline Au in a mixture of ethylene glycol and 1.5 M ZnCl2, for the efficient electrochemical reduction of CO2 to CO. The formed NP Au-Zn was further thermally treated with H2SO4, whereupon a significant enhancement of its electrocatalytic activity was achieved. The resulted NP Au exhibited over 65 times higher current density compared to a polycrystalline Au surface, and three times higher current density than NP Au-Zn at -0.6 V vs. RHE in a CO2-saturated 0.1 M NaHCO3 solution. In addition, the Faradaic efficiency (FE) of the formed NP Au at the applied potentials of -0.6 V for 10 h was extremely high (95.9%), revealing that the surface structure played a critical role in the electrochemical reduction of CO2. The superior catalytic activity, high production rate, excellent FE, and high stability make the unique NP Au developed in this study a promising electrocatalyst for the efficient and selective conversion of CO2 to CO.