Room temperature phase instability of cubic CsPbI₃ perovskite nanocrystals is one of the notorious limitations for practical applications in optoelectronic fields. Herein, the incorporation of Ni²⁺ ions into CsPbI₃ lattice was successfully achieved by a modified hot-injection method using nickel acetate as doping precursor. The as-prepared Ni²⁺ (3.3 mol%): CsPbI₃ nanocrystals exhibited an improved, near-unity (95%∼100%) photoluminescence quantum yield owing to the enhanced radiative decay rate and the decreased non-radiative decay rate. Additionally, Ni-doping was demonstrated to stabilize CsPbI₃ lattice and the Ni²⁺: CsPbI₃ film and colloidal solution can retain their red luminescence up to 15 days and 7 months in atmosphere, respectively. First-principle calculations verified that the significantly improved optical performance and stability of Ni²⁺: CsPbI₃ nanocrystals arose primarily from the increased formation energy due to the successful doping of Ni²⁺ in CsPbI₃. Benefiting from such an effective doping strategy, the as-prepared Ni²⁺-doped CsPbI₃ perovskite nanocrystals can function well as efficient red-light emitter toward the fabrication of high-performance perovskite LED with a peak external quantum efficiency of ∼7%.