AbstractPotassium‐ion batteries (KIBs) can offer high energy density, cyclability, and operational safety while being economical due to the natural abundance of potassium. Utilizing graphite as an anode, suitable cathodes can realize full cells. Searching for potential cathodes, this work introduces P3‐type K_0.5Ni_1/3Mn_2/3O₂ layered oxide as a potential candidate synthesized by a simple solid‐state method. The material works as a 3.2 V cathode combining Ni redox at high voltage and Mn redox at low voltage and exhibits highly reversible K⁺ ion (de)insertion at ambient and elevated (40–50 °C) temperatures. First‐principles calculations suggest the ground state in‐plane Mn–Ni ordering in the MO₂ sheets is strongly correlated to the K‐content in the framework, leading to an interwoven and alternative row ordering of Ni–Mn in K_0.5Ni_1/3Mn_2/3O₂. Postmortem and electrochemical titration reveal the occurrence of a solid solution mechanism during K⁺ (de)insertion. The findings suggest that the Ni addition can effectively tune the electronic and structural properties of the cathode, leading to improved electrochemical performance. This work provides new insights in the quest to develop potential low‐cost Co‐free KIB cathodes for practical applications in stationary energy storage.