Lithium-ion batteries (LIBs) have made tremendous successes in powering electronics and electric vehicles.  Yet, LIBs face an insurmountable challenge in sustainability for large-scale stationary applications. Lithium, without effective recycling strategies, is too rare to be used at the current pace, rendering a concern over this great technology.  Efforts are, therefore, guaranteed to pursue inexpensive and sustainable energy storage solutions.  As of today, Na-ion batteries have received tremendous attention; however, it faces its own hurdle before commercialization, which originated from the choice of anodes.  Alike sodium and potassium, magnesium is a benign and extremely Earth abundant element.  So far, most electrodes for Mg²⁺ ion storage have been studied in non-aqueous electrolytes, where intrinsic properties of Mg²⁺ ion storage might be covered up by factors, including instability of the electrolyte and large overpotentials of Mg metal counter/reference electrode.  To date, Mg²⁺ ion electrodes are still limited to Chevrel compounds or metal oxides.  Here, we, for the first time, report reversible Mg²⁺ ion storage properties in 3,4,9,10-perylenetetracaboxilicdianhydride (PTCDA) in an aqueous electrolyte.  In the potential range of -0.6 to 0.2 V vs Ag/AgCl reference electrode, PTCDA is able to deliver a reversible specific capacity of 125 mAh g^-1, a good rate capability with 75 mAh g^-1 at 500 mA g^-1 and a reasonable cycling stability for 100 cycles.  This work opens up a new avenue to harvest high-density charge storage for Mg²⁺ ions in an aqueous medium, which presents a large potential in magnesium ion batteries.