Research has recently been focused on high-performance next-generation batteries to replace secondary batteries due to capacity limitations and safety concerns. The Mg secondary battery is one candidate to realize high energy density storage batteries for practical applications. Ni and Co typically exhibit desirable electrochemical characteristics; therefore, we have attempted to synthesize new rock-salt compositions, MgxNiyCozO2, (x + y + z <= 2.0), as cathode materials for Mg rechargeable batteries, and investigated their crystal structures and electrochemical characteristics. The materials were synthesized by the reverse coprecipitation method. Powder X-ray diffraction and transmission electron microscopy analyses showed the obtained samples were a single phase of the rock-salt structure with the space group Fm (3) over barm. The vacancies at the metal sites were estimated by Rietveld analysis to determine the new chemical composition of MgxNiyCoz square 2-x-y-zO2 (0.41 < x < 0.64, 0.82 < y < 1.23, 0.24 < z < 0.61). Charge-discharge tests indicated the discharge characteristics varied according to the Mg composition and the Ni/Co ratio. The Co and Mg compositions were considered to facilitate the insertion/deinsertion of Mg2+. The present new material has the potential to be a superior cathode material for Mg secondary batteries by first-principles calculations.