In this work, we present tunnel-structured hollandite-type MnO2 as a reversible insertion cathode material for secondary Mg-ion batteries. Hollandite MnO2 [Hol] and the hollandite MnO2/acetylene black [Hol/AB] composite are synthesized as working electrodes. The reversible insertion/extraction of a Mg ion in an active host lattice in the potential window of - 1.8 to 1.0 V vs. Ag/Ag + at a current density of 100 mA/g is examined. In the case of hollandite MnO2/acetylene black composite, a specific discharge capacity of 210 mAh/g in the 1st cycle is observed upon the electrochemical insertion of ∼ 0.39 Mg/Mn. However, hollandite MnO2 exhibits a specific discharge capacity of 85 mAh/g upon the electrochemical insertion of 0.16 Mg/Mn under the same conditions. The tunnel framework of the hollandite MnO2 is retained with minor displacive adjustments even after substantial Mg-ion insertion/extraction. A reversible loss in capacity with increasing current density appears to be associated with a diffusion-limited transfer of the Mg ion in the solid state.