In the title complex, [Ni(H(2)O)(6)](C(6)H(10)N(2)O(6)PS)(2) x 6 H(2)O, the asymmetric unit consists of one-half of an Ni atom (which lies on an inversion centre) with three coordinated water molecules, one complete 2-carboxylato-2-(isothiouronium-S-ylmethyl)propane-1,3-diyl phosphate anion and three noncoordinated water molecules. The hexaaquanickel(II) cations have distorted octahedral coordination and are connected via water chains to form two-dimensional supramolecular networks parallel to the ab plane. The phosphate ester anion is linked via N-H...O and O-H...O hydrogen bonds, thus creating various ring, dimer and chain hydrogen-bonding patterns, and building up a second two-dimensional supramolecular network parallel to the ab plane. The crystal structure is further stabilized by an intra- and interlayer hydrogen-bond network. This work illustrates that a carboxylate with a caged phosphate ester can open its ring in the presence of dichloridotetrakis(thiourea)nickel, and the resulting polyfunctional anion can be used for constructing a complex hydrogen-bonding scheme.