In the title compound, [Cu2(OH)2(C12H8N2)2(H2O)2](C12H6O4)·6H2O, the two hydroxide groups bridge the two CuIIcations, forming a centrosymmetric binuclear complex cation, in which the CuIIcation is coordinated by a 1,10-phenanthroline (phen) molecule, one water molecule and two bridging hydroxide O atoms in a distorted N2O3square-pyramidal coordination geometry. The naphthalene-2,6-dicarboxylate anion is also located on an inversion centre. In the crystal, O—H...O hydrogen bonds link the cations, anions and lattice water molecules into a three-dimensional supramolecular architecture. Extensive π–π stacking is observed between the parallel or nearly parallel aromatic rings of adjacent phen ligands and naphthalenedicarboxylate anions, the centroid-to-centroid distances ranging from 3.4990 (16) to 3.8895 (16) Å.