We have reinvestigated the polyanionic compound Na₂Fe₂(C₂O₄)₃·2H₂O, previously reported to be electrochemically inactive in lithium-ion batteries (LIBs), as a positive electrode for sodium-ion batteries (NIBs). The present study demonstrates that it is capable of delivering a reversible capacity close to its theoretical value (117 mAhg^–1) with three redox plateaus at 2.9, 3.3 and 3.6 V vs. Na/Na⁺ in the potential range 1.7–4.2 V. The obtained energy density of 326 WhKg^–1 is among the highest of all reported polyanionic cathodes in NIBs. The origin of the electrochemical activity can be traced back to the electronic structure of the compound and the low migration energy barrier of the alkali ion observed in first-principles density-functional theory calculations.