The title compound, [Cu(C7H3NO5)(C12H8N2)(H2O)]·4.5H2O or [Cu(hypydc)(phen)(H2O)]·4.5H2O (phen is 1,10-phenanthroline and hypydcH2 is 4-hydroxypyridine-2,6-dicarboxylic acid), was obtained by the reaction of copper(II) nitrate hexahydrate with the proton-transfer compound (phenH)2(hypydc) in aqueous solution. Both the cationic and the anionic fragments of the proton-transfer compound are involved in complexation. Each CuII atom has a distorted octahedral geometry. It is hexacoordinated by three O atoms and three N atoms, from one phen fragment (as bidentate ligand), one (hypydc)2− unit (as tridentate ligand) and a water molecule. In the crystal structure, O—H...O and C—H...O hydrogen bonds, and π–π stacking interactions [centroid-to-centroid distance 3.5642 (11) Å] between the phen ring systems, contribute to the formation of a three-dimensional supramolecular structure.