The electrocatalytic reduction reaction of nitrate (NO 3 - ) to ammonia (NH 3 ) is a feasible way to achieve artificial nitrogen cycle. However, the low yield rate and poor selectivity towards NH 3 product is a technical challenge. Here we present a graphdiyne (GDY)-based tandem catalyst featuring Cu/Cu x O nanoparticles anchored to GDY support (termed Cu/Cu x O/GDY) for efficient electrocatalytic NO 3 - reduction. We achieve a high NH 3 yield rate of 25.4 mg h -1 mg cat. -1 (25.4 mg h -1 cm -2 ) with a Faradaic efficiency of 99.8% at an applied potential of -0.8 V versus RHE using the designed catalyst. These performance metrics outperform most reported NO 3 - to NH 3 catalysts in the alkaline media. Electrochemical measurements and density functional theory reveal that the NO 3 - preferentially attacks Cu/Cu x O, and the GDY can effectively catalyze the reduction of NO 2 - to NH 3 . This work highlights the efficacy of GDY as a new class of tandem catalysts for the artificial nitrogen cycle and provides powerful guidelines for the design of tandem electrocatalysts. This article is protected by copyright. All rights reserved.