K-deficient layered manganese-based oxides P2-K0.21MnO2 and P3-K0.23MnO2 were used as cathode materials of PIBs for the first time, and they delivered high discharge capacity and outstanding capacity retention rate. Display omitted Potassium-ions batteries (PIBs) are attracting increasing attention as up-and-coming youngster in large-scale grid-level energy storage benefiting from its low-cost and high energy density. Nevertheless, enough researches regarding indispensable cathode materials for PIBs are badly absent. Herein, we synthesize K-deficient layered manganese-based oxides (P2-K0.21MnO2 and P3-K0.23MnO2) and investigate them as cathode of PIBs for the first time. As the newcomer of potassium-containing layered manganese-based oxides (KxMnO2) group, P2-K0.21MnO2 delivers high discharge capacity of 99.3 mAh g−1 and P3-K0.23MnO2 exhibits remarkable capacity retention rate of 75.5%. Besides, in-situ XRD and ex-situ XRD measurements reveal the reversible phase transition of P2-K0.21MnO2 and P3-K0.23MnO2 with the potassium-ions extraction and reinsertion, respectively. This work contributes to a better understanding for the potassium storage in K-deficient layered KxMnO2 (x ≤ 0.23), possessing an important basic scientific significance for the exploitation and application of layered KxMnO2 in PIBs.