The title complex, [CoNa(CO5P)(H2O)5]n, was obtained by reacting sodium phosphono-formate with cobalt nitrate. The complex contains cobalt(II) and sodium ions, which are bridged by the O atoms of two aqua ligands. The Co(II) ion is octahedrally coordinated by three phosphonoformato ligands (one bi- and the other monodentate) and by two O atoms from the bridging aqua ligands. The sodium cation is hexa-coordinated by six O atoms from four bridging and two terminal aqua ligands. The complex mol-ecules are linked to give a three-dimensional structure by phosphono-formate ligands bridging Co(II) atoms and water mol-ecules establishing cobalt-sodium bridges. O-H⋯O hydrogen bonding between the aqua ligands and all O atoms of the phosphono-formato ligand and neighbouring aqua ligands help to consolidate the packing.