We report KFe(SO4)2 as a novel cathode material for potassium-ion batteries. The synthesis of phase-pure KFe(SO4)2 is confirmed through Rietveld refinement of X-ray diffraction data, and the ion diffusion path and possible positions of K+ ions in the crystal structure are verified using bond-valence sum analysis. In addition, the theoretical average voltage, ion diffusion paths, and associated activation barrier energies obtained by first-principles calculations predict feasibility of the KFe(SO4)2 as a robust cathode for potassium-ion batteries. . Experimentally, the average working voltage of KFe(SO4)2 is approximately 3.3 V (vs. K+/K) assisted by Fe3+/2+ redox pair, and the resulting specific capacity (94 mAh g−1) obtained at C/20 approaches to the theoretical capacity, 94 mAh g−1. In addition, the capacity retention of KFe(SO4)2 at 2C was approximately 80% of the initial capacity after 300 cycles, with a Coulombic efficiency of over 99%. X-ray absorption near-edge structure and operando synchrotron X-ray diffraction analyses reveals the reversible change of the crystal structure with a minimal volume difference (1.83%) by occurrence of Fe3+/2+ redox reaction during dis/charge. The demonstrated excellent structural stability of KFe(SO4)2 endorses feasibility of the KFe(SO4)2 as a sustainable cathode for potassium-ion batteries.