We report monoclinic Fe2(SO4)3 as a novel high-voltage cathode material of Na-ion batteries. The detailed structural information of monoclinic Fe2(SO4)3 are verified through Rietveld refinement and bond-valance sum energy mapping with X-ray diffraction analyses. Monoclinic Fe2(SO4)3 delivers a large specific capacity of ~120 mAh g−1 corresponding to ~1.8 mol Na intercalation into the structure and it exhibits the average operation voltage of ~3.25 V (vs. Na+/Na). Even at 5C, monoclinic Fe2(SO4)3 exhibits retention of ~76% of the capacity measured at C/10, indicating its outstanding power capability. Moreover, monoclinic Fe2(SO4)3 maintains up to ~78% of the initial capacity at 1C over 1000 cycles, with a high Coulombic efficiency of ~99%. This finding demonstrates the excellent cyclability of monoclinic Fe2(SO4)3, which results from the small volume change (~1%) during the Na de/intercalation process.