The title compound, poly[μ3-formato-μ3-hydroxo-cobalt(II)], [Co(CHO2)(OH)]n, adopts a polymeric three-dimensional network structure arising from the μ3-bridging mode of both the formate and the hydroxide. The Co atom lies in a fac octahedron of six O atoms, three of which belong to three different formates and the other three to different hydroxides.