In the title polymeric compound, [Co(C8H5O3)(2)(C4H4N2)(H2O)(2)](n), the Co-II atom is located on a twofold rotation axis and has a slightly distorted octahedral coordination sphere. In the equatorial plane, it is coordinated by two carboxylate O atoms of two symmetry-related monodentate formylbenzoate anions and by two N atoms of two bridging pyrazine ligands. The latter are bisected by the twofold rotation axis. The axial positions are occupied by two O atoms of the coordinating water molecules. In the formylbenzoate anion, the carboxylate group is twisted away from the attached benzene ring by 7.50 (8)degrees, while the benzene and pyrazine rings are oriented at a dihedral angle of 64.90 (4)degrees. The pyrazine ligands bridge the Co-II cations, forming linear chains running along the b-axis direction. Strong intramolecular O-H center dot center dot center dot O hydrogen bonds link the water molecules to the carboxylate O atoms. In the crystal, weak O-H-water center dot center dot center dot O-water hydrogen bonds link adjacent chains into layers parallel to the bc plane. The layers are linked via C-H-pyrazine center dot center dot center dot O-formyl hydrogen bonds, forming a three-dimensional network. There are also weak C-H center dot center dot center dot pi interactions present.