Provided is a cathode material for a rechargeable magnesium battery, represented by the chemical formula of Ag2SxSe1-x (0≤x≤1), a highly stable cathode material and a rechargeable magnesium battery including the same. The cathode material for a rechargeable magnesium battery has a higher discharge capacity and higher discharge voltage as compared to a typical commercially available cathode material, Chevrel phase, and shows excellent stability in an electrolyte for a rechargeable magnesium battery including chloride ions. In addition, after evaluating the cycle life of the cathode material, the cathode material shows an excellent discharge capacity per unit weight after 500 charge/discharge cycles, and thus is useful for a cathode material for a rechargeable magnesium battery.