In the title compound, C(10)H(12)N(8)O(4)·H(2)O, prepared from the reaction of glycoluril with paraformaldehyde, the organic molecule has mm symmetry. The asymmetric unit comprises one quarter of the mol-ecule and a half-mol-ecule of water. The dimer is formed by bridging two glycoluril mol-ecules with methyl-ene groups at the 1 and 6 positions. In the crystal structure, mol-ecules are linked via N-H⋯O and O-H⋯O hydrogen bonds, forming a two-dimensional framework.