In the title complex, [CoK(2)(C(3)H(2)O(4))(2)(H(2)O)(4)](n), the Co atom is located on a position with site symmetry 2/m, the K atom and one water mol-ecule are located on a mirror plane, and the malonate and one water mol-ecule are located on a twofold rotation axis. The K(I) atom is seven-coordinated by four carboxyl-ate O atoms from four malonate ligands and by three water O atoms, forming a distorted polyhedron. The Co(II) atom is in an almost octa-hedral environment formed by four carboxyl-ate O atoms from two malonate ligands and two water O atoms. The structure consists of layers parallel to (20[Formula: see text]) built up from edge-sharing KO(7) and CoO(6) polyhedra, which are connected by O-H⋯O hydrogen bonding including water mol-ecules into a three-dimensional network.