We report a monoclinic Na_2.4V₂(PO₄)₃/poly(3,4-ethylenedioxythiophene) (M-NVP/PEDOT) composite as a novel cathode material for sodium-ion batteries. The active materials were homogenously encapsulated by thin layers of the electro-conductive polymer PEDOT, and X-ray diffraction analysis indicated that the final products were crystallized into a monoclinic structure. The composite exhibited much more outstanding power capability and cyclability than pristine M-NVP. The discharge capacity of the M-NVP/PEDOT electrode was maintained up to ∼109.8 mA h g⁻¹ at 10C (1C = ∼178 mA g⁻¹), which was ∼9 times higher than that of pristine M-NVP under the same conditions. Moreover, whereas severe capacity degradation was observed for pristine M-NVP, the M-NVP/PEDOT composite delivered ∼81% capacity retention compared with the initial capacity over ∼500 cycles at 1C. In particular, the available capacity of M-NVP/PEDOT was ∼143.3 mA h g⁻¹, corresponding to ∼2.4 mol Na⁺ (de)intercalation, which is larger than the reversible capacity of rhombohedral Na₃V₂(PO₄)₃, at which only 2 mol Na⁺ can be (de)intercalated. This unexpectedly high capacity of the M-NVP/PEDOT composite was clearly confirmed through combined studies using several experiments and first-principles calculation.