The asymmetric unit of the title compound, [Ni(H2O)6](H2P2O6), contains one-half of the hexa-aqua-nickel(II) cation and one-half of the di-hydrogen hypodiphosphate anion. In the complex cation, the Ni(2+) atom is located on an inversion center and has an octa-hedral coordination sphere. The P-P distance in the centrosymmetric anion is 2.1853 (7) Å. In the crystal, discrete [Ni(H2O)6](2+) cations and (H2P2O6)(2-) anions are stacked in columns parallel to the c axis and are linked into a three-dimensional network by medium-strength O-H⋯O hydrogen bonds.