In the title compound, [Mg(H(2)O)(6)](C(7)H(4)NO(4))(2), a single six-coordinate Mg(2+) cation (site symmetry 2/m) is bonded to six water mol-ecules in a distorted octa-hedral geometry. The crystal packing between the complex cation and the zwitterionic organic cation (m symmetry) is stabilized by inter-molecular O-H⋯O hydrogen bonds and weak inter-molecular C-H⋯O inter-actions.