Nitrogen fixation under mild conditions has been one of the most important issues and a long-standing challenge in chemistry. By means of density functional theory (DFT) calculations, an experimentally available 2D MoC₆ was discovered as a nitrogen reduction reaction (NRR) electrocatalyst. Our results show that MoC₆ with high stability can be prepared experimentally. In particular, MoC₆ exhibits excellent catalytic activity for N₂ fixation at room temperature with a low potential of −0.54 V. The optimal active site, high utilization, selective stabilization of N₂H* species and destabilization of the species is responsible for the high activity of MoC₆. Our findings provide a rational strategy for nitrogen activation and ammonia production.