Renewable-energy-driven nitrate (NO₃⁻) electroreduction to ammonia (NH₃) (NERA) has been an attractive technology for decarbonizing NH₃ production and wastewater treatment. Improving NERA efficiency requires electrocatalysts that are earth-abundant and show fantastic performance. Here we report a semiempirical activity descriptor of e_g occupancy (of surface B-site cations) for identifying inexpensive perovskite oxides with extremely high efficacy toward NERA. We establish the descriptor by systematic investigations of more than 10 perovskite oxides. These investigations demonstrate that their intrinsic NERA activities display a volcano-shaped dependence on e_g occupancy and the optimized intrinsic activities are accessible at near-1 e_g occupancies. This could plausibly be attributed to the favorable overlaps between surface adsorbates and vertically-oriented e_g orbitals. More importantly, utilizing this descriptor, we predict a highly active, selective, and durable NERA electrocatalyst with a composition of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF). Because of its close-to-1 e_g occupancy (i.e. ~ 1.2), the BSCF features a superior NH₃ production rate of 0.12 g·h⁻¹·mg_cat.⁻¹ (Faradaic efficiency of 97.8%) that is at top of the volcano plot, and substantially outperforms most NERA electrocatalysts reported in literature.