AbstractUnderstanding the relationship between the electronic state of active sites and N₂ reduction reaction (NRR) performance is essential to explore efficient electrocatalysts. Herein, atomically dispersed Fe and Mo sites are designed and achieved in the form of well‐defined FeN₄ and MoN₄ coordination in polyphthalocyanine (PPc) organic framework to investigate the influence of the spin state of FeN₄ on NRR behavior. The neighboring MoN₄ can regulate the spin state of Fe center in FeN₄ from high‐spin (d_xy²d_yz¹d_xz¹¹¹) to medium‐spin (d_xy²d_yz²d_xz¹¹), where the empty d orbitals and separate d electron favor the overlap of Fe 3d with the N 2p orbitals, more effectively activating N≡N triple bond. Theoretical modeling suggests that the NRR preferably takes place on FeN₄ instead of MoN₄, and the transition of Fe spin state significantly lowers the energy barrier of the potential determining step, which is conducive to the first hydrogenation of N₂. As a result, FeMoPPc with medium‐spin FeN₄ exhibits 2.0 and 9.0 times higher Faradaic efficiency and 2.0 and 17.2 times higher NH₃ yields for NRR than FePPc with high‐spin FeN₄ and MoPPc with MoN₄, respectively. These new insights may open up opportunities for exploiting efficient NRR electrocatalysts by atomically regulating the spin state of metal centers.