Cu-based nanomaterials have received increasing interests for electrocatalytic applications in the CO₂ reduction reaction. It is however challenging to design nanostructured Cu electrodes to improve both the chemical kinetics and molecular transport under the reaction conditions. Here we report on a new type of three-dimensional, hierarchical Cu nanostructures as advanced electrocatalysts for CO₂ reduction. Driven by thermal oxidation, CuO nanowires and/or porous structures are grown on commercial Cu foams with three-dimensional (3D) frameworks. An electrochemical method is used to reduce CuO to Cu with the structural features largely preserved. The derived hierarchical Cu nanostructures are demonstrated to be highly active and selective for CO₂ reduction, achieving >80% Faradaic efficiency and ~3 times of enhancement in terms of CO₂ conversion rate as compared to the Cu nanowires grown on planar electrodes. Our work highlights the great potential of 3D Cu nanostructures for improving the energy efficiency and power performance of CO₂ electrolysis.