In the centrosymmetric dinuclear anions of the title bimetallic complex, {[Mg(H(2)O)(6)][Cu(2)(C(8)H(2)NO(7))(2)]·2H(2)O}(n), each Cu(II) ion is strongly coordinated by four O atoms in a distorted square-planar geometry. Two of these O atoms belong to phenolate groups and the other two to carboxylate groups from 5-nitro-2-oxidoisophthalate (L1) trianions, derived from 5-nitrobenzene-1,2,3-tricarboxylic acid (O(2)N-H(3)L). The phenolate O atoms bridge the two Cu(II) ions in the anion. In addition, each Cu(II) cation interacts weakly with a symmetry-related carboxylate O atom of an adjacent L1 ligand, giving a square-pyramidal coordination geometry. The copper residue forms a ladder-like linear coordination polymer via L1 ligands. The [Mg(H(2)O)(6)](2+) cations sit on centres of inversion. The polymeric anions, cations and free water molecules are self-assembled into a three-dimensional supramolecular network via O-H···O hydrogen bonds.