Herein, P2‐type layered manganese and ruthenium oxide is synthesized as an outstanding intercalation cathode material for high‐energy density Na‐ion batteries (NIBs). P2‐type sodium deficient transition metal oxide structure, Na_0.67Mn_1‐xRu_xO₂ cathodes where x varied between 0.05 and 0.5 are fabricated. The partially substituted main phase where x = 0.4 exhibits the best electrochemical performance with a discharge capacity of ≈170 mAh g⁻¹. The in situ X‐ray Absorption Spectroscopy (XAS) and time‐resolved X‐ray Diffraction (TR‐XRD) measurements are performed to elucidate the neighborhood of the local structure and lattice parameters during cycling. X‐ray photoelectron spectroscopy (XPS) revealed the oxygen‐rich structure when Ru is introduced. Density of States (DOS) calculations revealed the Fermi‐Level bandgap increases when Ru is doped, which enhances the electronic conductivity of the cathode. Furthermore, magnetization calculations revealed the presence of stronger Ru─O bonds and the stabilizing effect of Ru‐doping on MnO6 octahedra. The results of Time‐of‐flight secondary‐ion mass spectroscopy (TOF‐SIMS) revealed that the Ru‐doped sample has more sodium and oxygenated‐based species on the surface, while the inner layers mainly contain Ru–O and Mn–O species. The full cell study demonstrated the outstanding capacity retention where the cell maintained 70% of its initial capacity at 1 C‐rate after 500 cycles.