The electrochemical CO₂ reduction reaction (CO₂RR) to produce high value-added hydrocarbons and oxygenates presents a sustainable and compelling approach toward a carbon-neutral society. However, uncontrollable migration of active sites during the electrochemical CO₂RR limits its catalytic ability to simultaneously achieve high C₂ selectivity and ultradurability. Here, we demonstrate that the generated interfacial CuAlO₂ species can efficiently stabilize the highly active sites over the Cu–CuAlO₂–Al₂O₃ catalyst under harsh electrochemical conditions without active sites regeneration for a long-term test. We show that this unique Cu–CuAlO₂–Al₂O₃ catalyst exhibits ultradurable electrochemical CO₂RR performance with an 85% C₂ Faradaic efficiency for a 300 h test. Such a simple interfacial engineering design approach unveiled in this work would be adaptable to develop various ultradurable catalysts for industrial-scale electrochemical CO₂RR.