Single crystals of the title compound, [Co(C2O4)(H2O)2]n, have been prepared by hydrothermal methods and characterized by X-ray diffraction analysis. The crystal structure consists of infinite one-dimensional chains of diaquacobalt(II) units bridged by oxalate groups. These chains lie on twofold symmetry axes parallel to the b axis, and the [Co(C2O4)]n system is nearly planar within experimental error. The cobalt(II) coordination polyhedra are irregular octahedra, with oxalate O atoms at the equatorial positions and water molecules at the axial positions. The chains are linked by hydrogen bonds via the water molecules.