AbstractStructural engineering of nanomaterials offers a promising way for developing high‐performance catalysts toward catalysis. However, the delicate modulation of thermodynamically unfavorable nanostructures with unconventional phases still remains a challenge. Here, the synthesis of hierarchical AuCu nanostructures is reported with hexagonal close‐packed (2H‐type)/face‐centered cubic (fcc) heterophase, high‐index facets, planar defects (e.g., stacking faults, twin boundaries, and grain boundaries), and tunable Cu content. The obtained 2H/fcc Au₉₉Cu₁ hierarchical nanosheets exhibit excellent performance for the electrocatalytic CO₂ reduction to produce CO, outperforming the 2H/fcc Au₉₁Cu₉ and fcc Au₉₉Cu₁. The experimental results, especially those obtained by in‐situ differential electrochemical mass spectroscopy and attenuated total reflection Fourier‐transform infrared spectroscopy, suggest that the enhanced catalytic performance of 2H/fcc Au₉₉Cu₁ arises from the unconventional 2H/fcc heterophase, high‐index facets, planar defects, and appropriate alloying of Cu. Impressively, the 2H/fcc Au₉₉Cu₁ shows CO Faradaic efficiencies of 96.6% and 92.6% at industrial current densities of 300 and 500 mA cm⁻², respectively, as well as good durability, placing it among the best CO₂ reduction electrocatalysts for CO production. The atomically structural regulation based on phase engineering of nanomaterials (PEN) provides an avenue for the rational design and preparation of high‐performance electrocatalysts for various catalytic applications.