We report for the first time a zero-strain cathode, Na₄Fe₇(PO₄)₆, for sodium–ion batteries (SIBs). This new iron-based polyanionic cathode delivers a reversible capacity of 66.5 mA h g⁻¹ at 5 mA g⁻¹ with almost 100% capacity retention over 1000 cycles under 200 mA g⁻¹, and the outstanding performance benefits from single-phase-transition processes with a tiny volume change of only ∼0.24%.