The title compound, [Ni(H2O)6][BaCr(C2O4)3(H2O)3]2·4H2O, was obtained in the form of single crystals from the slow evaporation of an aqueous mixture of {Ba6(H2O)17[Cr(C2O4)3]4}·7H2O and NiSO4·6H2O in the molar ratio 1:4. Its structure is made up of corrugated anionic (101) layers of formula [BaCr(C2O4)3(H2O)3] n n − that leave voids accommodating the charge-compensating cations, [Ni(H2O)6]2+ (point group symmetry \overline{1}), as well as the water molecules of crystallization. The anionic layers are built from the connection of barium and chromium atoms through bridging oxalate ligands. The CrIII atom is hexacoordinated by O atoms of three oxalate ligands while the BaII atom is tenfold coordinated by three O atoms of water molecules and seven O atoms of four oxalate ligands. Each NiII atom sits on an inversion center and is coordinated by six water molecules. One of the uncoordinated water molecules is disordered over two sites, with a refined occupancy ratio of 0.51 (5):0.49 (5). In the crystal, extensive O—H...O hydrogen-bonding interactions link the anionic layers, the charge-balancing cations as well as the water molecules of crystallization into a three-dimensional supramolecular network.