P2-type Na2/3Fe1/3Mn2/3O2 was prepared by means of solid-state reaction. Its electrochemical properties as a cathode for sodium-ion batteries were investigated in the 1.5–4.3 V range vs. Na metal. This material delivers an initial discharge capacity as high as 193 mAh/g and maintains 153 mAh/g in 40 cycles. Such a high reversible capacity is achieved because both Mn3+/Mn4+ and Fe3+/Fe4+ become active in the studied voltage range, as confirmed by X-ray absorption spectroscopy (XAS). The thermal stability of charged cathodes was investigated by differential scanning calorimetry (DSC). Although the total heat generated by the fully charged Na0.25Fe1/3Mn2/3O2 is larger than that of the Li0.5CoO2 used as a reference, it is remarkably stable up to ca. 320 °C.