The electrocatalytic nitrogen reduction reaction (NRR) is considered to be the most desirable strategy for ammonia production but still faces many challenges in terms of high activity and high selectivity. Based on density functional theory (DFT) calculations, the catalytic performance of a series of (3d, 4d and 5d series) transition metals atoms (TMs) anchored on a novel graphitic carbon–nitrogen (g-C₉N₄) monolayer has been systematically investigated. We find that TMs can bind tightly to g-C₉N₄ and form single-atom catalysts (SACs) with high thermodynamic stability. The four candidates, Nb, Ta, W and Re@g-C₉N₄, not only exhibit high NRR catalytic activity but also effectively inhibit the competitive HER. Among them, Nb@g-C₉N₄ is the most promising NRR catalyst with a lowest limiting potential of −0.21 V. The optimal reaction path for Nb, W and Re@g-C₉N₄ is via the enzymatic mechanism, while Ta@g-C₉N₄ tends to be through the distal mechanism. In addition, the decomposition potential of the g-C₉N₄ monolayer is higher than the limiting potential of all four SACs, ensuring the feasibility of the experimental implementation. This work identifies efficient NRR catalysts and provides a feasible screening scheme.