In the title compound, poly[[aqua(1,10-phenanthroline)cobalt(II)]-mu4-dihydrogen benzene-1,2,4,5-tetracarboxylato], [Co(C10H4O8)(C12H8N2)(H2O)]n, each cobalt(II) cation has an octahedral geometry completed by one aqua O atom, three carboxy O atoms belonging to three H2TCB2- anions (H2TCB2- is dihydrogen benzene-1,2,4,5-tetracarboxylate) and two N atoms from a 1,10-phenanthroline molecule. In the asymmetric unit, there are two half H2TCB2- anions lying about independent inversion centres. The bridging H2TCB2- anions have two coordination modes, viz. mu2-H2TCB2- and mu4-H2TCB2-, resulting in a two-dimensional coordination polymer. Furthermore, a three-dimensional network is formed by O(carboxy)...O(carboxy) hydrogen-bond interactions, with H...A distances in the range 1.69-1.82 A, and O(carboxy)...O(water) interactions, with H...A distances in the range 1.91-1.94 A.