High‐capacity O3‐type layered sodium oxides are considered one of the most promising cathode materials for the next generation of Na‐ion batteries (NIBs). However, these cathodes usually suffer from low high‐rate capacity and poor cycling stability due to structure deformation, native air sensitivity, and interfacial side reactions. Herein, a multi‐site substituted strategy is employed to enhance the stability of O3‐type NaNi_0.5Mn_0.5O₂. Simulations indicate that the Ti substitution decreases the charge density of Ni ions and improves the antioxidative capability of the material. In addition, the synergistic effect of K⁺ and Ti⁴⁺ significantly reduces the formation energy of Na⁺ vacancy and delivers an ultra‐low lattice strain during the repeated Na⁺ extraction/insertion. In situ characterizations verify that the complicated phase transformation is mitigated during the charge/discharge process, resulting in greatly improved structure stability. The co‐substituted cathode delivers a high‐rate capacity of 97 mAh g⁻¹ at 5 C and excellent capacity retention of 81% after 400 cycles at 0.5 C. The full cell paired with commercial hard carbon anode also exhibits high capacity and long cycling life. This dual‐ion substitution strategy will provide a universal approach for the new rational design of high‐capacity cathode materials for NIBs.