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Sodium-ion batteries (SIBs) are regarded as the most promising technology for large-scale energy storage systems. However, the practical application of SIBs is still hindered by the lack of applicable cathode materials. Herein, a novel phase-pure polyanionic Na8Fe5(SO4)9 is designed and employed as a cathode material for SIBs for the first time. The Na8Fe5(SO4)9 has an alluaudite-type sulfate framework and small Na+ ion diffusion barriers. As expected, the as-synthesized Na8Fe5(SO4)9@rGO exhibits a high working potential of 3.8 ​V (versus Na/Na+), a superior reversible capacity of 100.2 mAh g−1 at 0.2 ​C, excellent rate performance (∼80 mAh g−1 at 10 ​C, ∼63 mAh g−1 at 50 ​C), and an ultra-long cycling life (91.9% capacity retention after 10,000 cycles at 10 ​C, 81% capacity retention after 20,000 cycles at 50 ​C). We use various techniques and computational methods to comprehensively investigate the electrochemical reaction mechanisms of Na8Fe5(SO4)9@rGO.