A method for producing a Group III nitride crystal including preparing an RAMgO4 substrate containing a single crystal represented by the general formula RAMgO4 (wherein R represents one or a plurality of trivalent elements selected from the group consisting of Sc, In, Y, and a lanthanoid element, and A represents one or a plurality of trivalent elements selected from the group consisting of Fe(III), Ga, and Al); and growing a Group III nitride crystal containing Mg on the RAMgO4 substrate.