AbstractOxide‐derived Cu catalysts from Cu₂O microcrystals are capable of electrochemically converting CO₂ into various value‐added chemicals. However, their structural transformation and associated preferred products remain unclear, requiring further investigation. Herein, Cu₂O microcrystals with controllable low‐ and high‐index facets exposure are fabricated to differentiate the effects of initial exposed facets on their structural reconstruction and product selectivity in electrochemical CO₂ reduction reaction. Combined in situ characterizations and theoretical investigation reveal the direct correlations of Cu₂O reconstruction and product selectivity to its initial facet exposure. The Cu₂O low‐index facet, being more stable with a high energy barrier on material reduction, tends to partially maintain its original crystalline structure and larger Cu₂O particle size throughout the transformation. The derived flatter surface and limited Cu₂O/Cu interfaces result in a favorable selectivity toward 2‐electron transfer products. The chemically active Cu₂O high‐index facet (311) is energetically favorable to be reduced owing to the feasible protonation process, thus experiencing a drastic reconstruction with rich newly formed Cu nanoparticles and evolved fine Cu₂O grains; Such a reconstruction creates uncoordinated Cu species and abundant boundaries, benefiting charge transfer and increasing the local pH by confining OH⁻, thus leading to a high selectivity toward C₂₊ products.