Lithium nickel oxide (LiNiO₂) is a promising cobalt-free cathode material for lithium-ion batteries due to its high theoretical capacity and wide operating voltage.1 However, its practical performance is limited by structural instability arising from Ni/Li cation mixing and local disorder. In this work, we investigate the pressure-dependent structural and electronic evolution of LiNiO₂ in a diamond anvil cell using in situ synchrotron XRD at Psiché beamline (SOLEIL) and Raman spectroscopy up to 50 GPa. XRD analysis reveals that LiNiO₂ retains its rhombohedral (R̄3m) structure across the pressure range, with no evidence of phase transition. The lattice parameters and unit cell volume decrease smoothly with pressure, and fitting to the third-order Birch–Murnaghan equation of state yields a bulk modulus of ~153 GPa. Notably, the I(003)/I(104) intensity ratio initially increases up to ~5GPa, indicating reduced cation mixing and improved layered ordering. At higher pressures, this ratio decreases progressively, suggesting enhanced Li/Ni disorder. Peak broadening, particularly of the (104) reflection, further indicates anisotropic structural distortion and increasing in-plane disorder. Complementary Raman measurements show a pressure-induced blue shift of the Eg and A1g modes, consistent with lattice compression. Mode broadening, especially in the Eg band, reflects increasing structural disorder and non-stoichiometry linked to cation mixing. Overall, the results demonstrate a competition between ordering and disordering mechanisms under pressure. Initial compression stabilizes the layered structure, while higher pressures promote cation mixing and local distortions. This study highlights the critical role of pressure in tuning structural disorder and provides insight into the structure–property relationships in Ni-rich cathodes.