The title compound, [Ni(C7H3NO4)(H2O)4]H2O, synthesized by reaction of nickel(II) nitrate hexahydrate with pyridine- 2,5-dicarboxylic acid and piperazine in water. It has essentially the same structure with the analogous nickel(II) dihydrate complex [Shiu, Chen, Liao & Wang (2003). Acta Cryst. E59, m1072–m1074]. The compound contains a six-coordinate NiII ion, which is bonded to the N and an O atom of the carboxylate group in the 2-position of the pyridine-2,5- dicarboxylate ligand, and four water O atoms. The Ni^II^ atom has a distorted octahedral coordination environment. Intermolecular hydrogen-bonding interactions are present, linking the nickel(II) complex and water molecules in the crystal structure.
