In the title hydrated salt, 2NH4 +·C 12H12N4O6 2-·H2O, the two hexa-hydro-pyrimidine rings in the dianion are inclined to one another at a dihedral angle of 62.76 (5)°. In the crystal structure, the anions and water mol-ecules are linked into sheets parallel to the bc plane by inter-molecular O- H⋯O hydrogen bonds and sustained by C- H⋯O contacts. The linking of the anions and water mol-ecules with the cations by N- H⋯O hydrogen bonds creates a three-dimensional extended network. The crystal structure is further stabilized by very weak C- H⋯π inter-actions.