Electrocatalytic N₂ reduction reaction (NRR) represents an appealing solution for sustainable ammonia production, whereas exploring high-efficiency NRR catalysts is highly desired but extremely challenging. Herein, we combine Ti₃C₂T_x-MXene quantum dots (MQDs) with porous Cu nanosheets to design a novel heterostructured MQDs/Cu as an effective and durable NRR catalyst. Impressively, MQDs/Cu showed a synergistically enhanced NRR activity with an NH₃ yield of 78.5 μg h⁻¹ mg⁻¹ (−0.5 Ⅴ) and a Faradaic efficiency of 21.3% (−0.4 Ⅴ), far superior to pure MQDs and Cu, and outperforming the majority of the state-of-the-art NRR catalysts. Density function theory computations demonstrated that the synergy of MQDs and Cu enabled the creation of interfacial Cu-Ti dimer as dual-active-centers to strongly activate the absorbed N₂ and promote the *N₂H formation, consequently resulting in the much reduced energy barriers and greatly enhanced NRR performance.