Titanium dioxide bronze (TiO2-B) nanowires were prepared by the hydrothermal method and used as the positive electrode for Mg rechargeable batteries and Li+/Mg2+ hybrid-ion batteries. First-principles calculations showed that the diffusion barrier for Mg2+ (0.6 eV) in the TiO2-B lattice was more than twice of that for Li+ (0.3 eV). Electrochemical impedance spectroscopy showed that the charge transfer resistance of TiO2-B in the Mg2+ ion electrolyte was much larger than that in the Li+/Mg2+ hybrid electrolyte. For these reasons, the Mg rechargeable battery showed a small discharge capacity of 35 mAh g−1 resulting from an electrochemical double-layer capacitive process. In comparison, the TiO2-B nanowires exhibited a large capacity (242 mAh g−1 at the 20 mA g−1 current density), high rate capability (114 mAh g−1 at 1 A g−1), and excellent cycle stability in the Li+/Mg2+ hybrid-ion battery. The dominant reaction occurred in the TiO2-B electrode was intercalation of Li+ ions, of which about 74% of the total capacity was attributed to Li+ pseudo-capacitance.