Abstract: The electrocatalytic nitric oxide reduction reaction (NORR) converts nitric oxide (NO) into value-added ammonia (NH₃) and hydroxylamine (NH₂OH) under mild conditions, offering a promising route for pollutant valorization and green chemical synthesis. Carbon-supported Fe single-atom catalysts (SACs) exhibit high activity toward NORR, while their activity and selectivity can be regulated by tailoring the coordination microenvironment of the Fe center. Fe SACs are typically N₄-coordinated. Substituting N with less electronegative B, which contains vacant orbitals, creates an electronegativity gradient and breaks coordination symmetry, thereby reconstructing the local charge distribution at the Fe site. A series of carbon-supported FeN_xB_4−x SACs (x = 0−3, with α and β isomers for x = 2) is evaluated using density functional theory calculations combined with a hybrid explicit–implicit solvent model. The effects of the coordination microenvironment on NORR pathways, product selectivity, and activity are systematically elucidated. Gibbs free-energy analysis shows that the potential-determining step on all catalysts is the initial hydrogenation of *NO. The asymmetrically B/N-coordinated FeN₃B₁ and FeN₁B₃ catalysts exhibit the highest activities, and FeN₃B₁ has a limiting potential of −0.61 V. Projected density-of-states analysis indicates that increasing the B/N coordination ratio first shifts the Fe d-band center (ε_d) downward when one or two B atoms are introduced and then upward when three or four B atoms are present, thereby weakening and subsequently strengthening intermediate adsorption. A volcano relationship is obtained between ε_d and the NORR limiting potential, demonstrating that optimal activity requires an intermediate adsorption strength. The Fe ε_d is therefore identified as a key activity descriptor, and asymmetric B/N coordination is established as a design principle for efficient NORR catalysts.