<p>Rechargeable magnesium batteries (RMBs) have faced challenges in utilizing oxide cathodes due to the inherently sluggish Mg diffusion and poor compatibility with electrolytes, despite the high redox potential. Herein, we present a prototype RMB that is operational at room temperature, consisting of a nanoparticulate amorphous oxide cathode, fluorinated alkoxyborate (Mg[B(HFIP)₄]₂) as the electrolyte, and a Mg metal anode. The amorphous Mg_<i>x</i>Ti_1/9Mo_2/9O cathode contains a considerable free volume formed by ion exchange between monovalent and divalent cations, facilitating Mg diffusion and eventually realizing reversible Mg insertion/extraction at room temperature. The reasonable compatibility of the present cathode with the electrolyte enables full cell operation with a Mg metal anode, and various analyses have demonstrated that Mg intercalation is responsible for the battery performance. The discharging capacity is ~150 mAh g⁻¹, and 70 mAh g⁻¹ is maintained after 200 cycles. These findings demonstrate the feasibility of RMBs with oxide cathodes that are operational at room temperature.</p>