Abstract Nitrate reduction to ammonia has attracted much attention for nitrate (NO 3 ‐ ) removal and ammonia (NH 3 ) production. Identifying promising catalyst for active nitrate electroreduction reaction (NO 3 RR) is critical to realize efficient upscaling synthesis of NH 3 under low‐temperature condition. For this purpose, by means of spin‐polarized first‐principles calculations, the NO 3 RR performance on a series of graphitic carbon nitride (g‐CN) supported double‐atom catalysts (denoted as M 1 M 2 @g‐CN) are systematically investigated. The synergistic effect of heterogeneous dual‐metal sites can bring out tunable activity and selectivity for NO 3 RR. Amongst 21 candidates examined, FeMo@g‐CN and CrMo@g‐CN possess a high performance with low limiting potentials of ‐0.34 and ‐0.39 V, respectively. The activities can be attributed to a synergistic effect of the M 1 M 2 dimer d orbitals coupling with the anti‐bonding orbital of NO 3 ‐ . The dissociation of deposited FeMo and CrMo dimers into two separated monomers is proved to be difficult, ensuring the kinetic stability of M 1 M 2 @g‐CN. Furthermore, the dual‐metal decorated on g‐CN significantly reduces the bandgap of g‐CN and broadens the adsorption window of visible light, implying its great promise for photocatalysis. This work opens a new avenue for future theoretical and experimental design related to NO 3 RR photo‐/electrocatalysts.