A new three-dimensional nickel–sodium compound of formula [Na₂Ni(mal)₂(H₂O)₆]_n (H₂mal=malonic acid) was prepared and its structure was determined by X-ray diffraction methods. Four malonate–oxygen atoms and two trans-(Ni) or cis-(Na) coordinated water molecules build distorted octahedral surroundings around the metal atoms. The malonate group exhibits bidentate (Ni) and tetrakis-monodentate (Na) coordination modes. The structure can be described as corrugated sheets of malonato-containing Ni(II) and Na(I) cations which grow in the (101) plane, each sheet being linked to the adjacent one in the [101] direction through bis(μ-aqua)disodium(I) units. Within each corrugated sheet, the nickel atoms define a square lattice, the edge and the diagonal being 8.895(3) and 12.579(3) Å, respectively. Variable-temperature magnetic susceptibility measurements (2.0–290 K) reveal the occurrence of a very weak antiferromagnetic interaction between the nickel(II) ions.