Electrochemical reduction of nitrogen (NRR) at ambient conditions using renewable energy is a green and sustainable strategy for NH3 synthesis, which is one of the most important chemicals and carbon-free carriers. Searching for low-cost, highly efficient, and stable NRR electrocatalysts is critical to achieve this goal. Herein, by means of comprehensive density functional theory (DFT) computations, we have designed a new class of NRR electrocatalyst based on single transition metal (TM) atom supported on the experimentally feasible two-dimensional C2N monolayer (TM@C2N). Our computations revealed that among all studied TM@C2N-based electrocatalysts, Mo@C2N possesses the best NRR catalytic performance due to its smallest onset potential of 0.17 V, which is even lower than the well-established stepped Ru(0001) surface (0.43 V). Remarkably, the NRR on the Mo@C2N prefers to proceed via the distal pathway, in which the hydrogenation of NH2* species to NH3(g) is the potential-determining step. The catalytic performance of these TM@C2N for the NRR can be well explained by their adsorption strength with N2H* species. Our findings open up a new avenue for optimizing TM catalytic performance for the NRR with the least metal atoms on porous low-dimensional materials.