This is the first report about a spinel manganese oxide that serves as a high capacity positive electrode material for the sodium ion battery. By electrochemically extracting Li from a monoclinic layered Li2MnO3, we prepared Li2−x MnO3 (x = 1.6-1.8) of which the Li-extracted domain has a cubic spinel structure. The reversible discharge and charge capacity of Li2−x MnO3 versus the Na negative electrode initially exceeded 200mA·h·g−1, suggesting that close to one molar equivalent of Na is inserted in and extracted from the formula unit, Li2−x MnO3. The electrode retains the capacity of 160mA·h·g−1 after 50 cycles. On the other hand, the Li2−x MnO3 electrode versus the Li negative electrode significantly degrades upon cycling. The ex-situ synchrotron X-ray diffraction (SR-XRD) and transmission electron microscopy (TEM) analyses revealed that the spinel domain of Li2−x MnO3 retains its crystallographic structure during the Na insertion and extraction, although the crystal significantly loses its periodicity when Na is inserted. A numerical simulation of the SR-XRD profile suggests that the Na-inserted Li2−x MnO3 has the periodicity of only one to two unit cells while retaining the spinel structure. XAS revealed that the reversible capacity is found to be dominated by the redox between Mn(III) and Mn(IV) and the Mn-Mn distance significantly loses its correlation upon Na-insertion, which is consistent with the broad SR-XRD profile.