Transition metal dichalcogenide materials have been considered as promising anode materials for rechargeable sodium-ion batteries because of their high specific capacity and low cost. Here, we demonstrate an iron sulfide Fe₃S₄ as a new anode material for a rechargeable sodium-ion battery. The involved conversion mechanism has been proved when the as-prepared Fe₃S₄ was used as the host material for sodium storage. Remarkably, a compound FeS_x with quantum size generated by conversion reaction overcame the kinetic and thermodynamic constraints of chemical conversion to achieve superior cycling and rate capability. As a result, the as-prepared Fe₃S₄ electrode delivers a high reversible specific capacity of 548 mA h g⁻¹ at 0.2 A g⁻¹, together with an excellent cycling stability of 275 mA h g⁻¹ after 3500 cycles at 20 A g⁻¹.