Potassium-ion batteries are a compelling technology for large scale energy storage due to their low-cost and good rate performance. However, the development of potassium-ion batteries remains in its infancy, mainly hindered by the lack of suitable cathode materials. Here we show that a previously known frustrated magnet, KFeC₂O₄F, could serve as a stable cathode for potassium ion storage, delivering a discharge capacity of ~112 mAh g⁻¹ at 0.2 A g⁻¹ and 94% capacity retention after 2000 cycles. The unprecedented cycling stability is attributed to the rigid framework and the presence of three channels that allow for minimized volume fluctuation when Fe²⁺/Fe³⁺ redox reaction occurs. Further, pairing this KFeC₂O₄F cathode with a soft carbon anode yields a potassium-ion full cell with an energy density of ~235 Wh kg⁻¹, impressive rate performance and negligible capacity decay within 200 cycles. This work sheds light on the development of low-cost and high-performance K-based energy storage devices.