The magnesium-metal battery, which consists of a cathode, a Mg-metal anode, and a nonaqueous electrolyte, is a safer and less expensive alternative to the popular Li-ion battery. However, the performance of Mg batteries is greatly limited by the low electrochemical oxidative stability of nonaqueous electrolytes, the slow Mg2+ diffusion into the cathode, and the irreversibility of Mg striping and plating on the Mg metal anode. Here, we report the first Mg-ion battery using a Mg2+ aqueous electrolyte, nickel hexacyanoferrate cathode, and polyimide anode. The operation depends on Mg2+ intercalation?deintercalation at the cathode and reversible enolization at the anode, accompanied by Mg2+ transport between cathode and anode. The cell exhibits a maximum cell voltage of 1.5 V and a supercapacitor-like high power, and it can be cycled 5000 times. This system points the way to improved Mg-based rechargeable batteries.