A novel layered P2–K~2/3[Ni1/3Mn2/3]O2 (K0·64Na0·04[Ni1/3Mn2/3]O2, KNMO) is presented as a high-performance cathode for K+-ion batteries. KNMO is prepared via an electrochemical ion-exchange of P2–Na0.64[Ni1/3Mn2/3]O2 (NNMO). During 50 charge/discharge (C/D) cycles within a P2-phase range, NNMO gradually turns to KNMO and finally reaches KNMO through a biphasic state. The as-prepared KNMO shows a P2–O2 transition at 4.65 V vs. K/K+, which implies that K+-preference in the prismatic sites of KNMO leads to the formation of O2-phases at higher voltages than that predicted from NNMO. Thanks to the retardation of the P2–O2 transition and fast diffusion during K+ extraction (0.57 × 10−11 cm2 s−1), KNMO can be quickly charged within the P2-phase region without a significant loss in discharge capacities. For example, comparable capacities of 82 and 75 mAh·g−1 during discharge (0.5C) are obtained after charging at 0.5 and 15C (2580 mA g−1), respectively. This behavior is contrasted with a significant reduction in capacities when cycled at the same C/D rates or when cycled at C/D rates of 0.5/15C, respectively. KNMO also shows excellent cyclability during fast charging, retaining a reversible capacity of 73 mAh·g−1 after 200C/D cycles (0.01 mAh·g−1·cycle−1).