We systematically investigated the catalytic performance of 3d, 4d, and 5d transition metals anchored onto two-dimensional extended porphyrin (PP) substrates as nitrogen reduction reaction (NRR) electrocatalysts, employing density functional theory (DFT) calculations and four-step high-throughput screening. Four novel metalloporphyrin (MPP, M = Zr, Nb, Hf, and Re) single-atom catalyst candidates have been identified due to their excellent catalytic performance (low onset potential, high stability, and selectivity). Through comprehensive reaction path search, the maximum Gibbs free energy changes for NRR on the ZrPP (enzymatic-consecutive hybrid path), NbPP (consecutive path), HfPP (enzymatic-consecutive hybrid path), and RePP (distal path) catalysts are 0.38, 0.41, 0.53, and 0.53 eV, respectively. Band structures, projected density of states, and charge/spin distributions show that the high catalytic activity is due to significant orbital hybridizations and charge transfer between N₂ and MPP catalysts. We hope our work will promote experimental synthesis of these NRR electrocatalysts and provide new opportunities to the electrochemical conversion of N₂ to NH₃ under ambient conditions.