In the title compound, {[Co(2)(C(16)H(6)O(8))(C(12)H(8)N(2))(H(2)O)(2)]·H(2)O}(n), one Co(II) ion has a {CoN(2)O(4)} distorted octa-hedral environment defined by two N atoms of one 1,10-phenanthroline (phen) ligand, three O atoms of the carboxyl-ate groups of three biphenyl-3,3',4,4'-tetra-carboxyl-ate (BPTC) ligands, one of which is bidentate, and one O atom from one coordinated water mol-ecule. The other Co(II) atom is surrounded by six O atoms from four different BPTC ligands and one coordinated water mol-ecule. Each BPTC ligand forms eight coordination bonds with seven Co(II) atoms, leading to a layer structure along the ac plane. Uncoordinated water mol-ecules occupy the space between the layers, and inter-act via inter-layer O-H⋯O hydrogen bonds along the b axis, generating a three-dimensional supra-molecular network.