Ammonia (NH3) is one of the most significant industrial chemical products due to its wide applications in various fields. However, the production of NH3 from the electrochemical nitrogen (N-2) reduction reaction (NRR) under ambient conditions is one of the most important issues that remain challenging for chemists. Herein, the candidacy of a series of molybdenum (Mo)-based single-atom catalysts (SACs) supported on N-doped black phosphorus (BP) as the electrocatalyst for the NRR has been evaluated by means of density functional theory (DFT) calculations. In particular, Mo1N3 has been found to chemically adsorb N-2, and it exhibits the highest catalytic activity toward the NRR with an ultralow overpotential of 0.02 V via the associative distal mechanism, indicative of catalyzing the NRR under ambient conditions. Additionally, Mo1N3 shows the fast removal of the produced NH3 with a free energy uphill of only 0.56 eV and good stability of NRR intermediates. Moreover, the Mo-based SACs were demonstrated to be more selective to the NRR over the competing hydrogen evolution reaction (HER) process. These excellent features render Mo1N3 on BP as a compelling highly efficient and durable catalyst for electrochemical N-2 fixation. Our results provide a rational paradigm for catalytic nitrogen fixation by SACs in two-dimensional (2D) materials under ambient conditions.