In the title hydrated salt, CH(6)N(3) (+)·C(4)H(5)O(6) (-)·H(2)O, the deprotonated carboxyl group is disordered over two positions with a site-occupancy ratio of 0.945 (3):0.055 (3). The bond lengths in the guanidinium cation are inter-mediate between normal C-N and C=N bond lengths, indicating significant delocalization in this species. In the crystal structure, anions and water mol-ecules are linked into sheets parallel to the ab plane by inter-molecular O-H⋯O hydrogen bonds. The linking of the anions and water mol-ecules with the cations by inter-molecular N-H⋯O hydrogen bonds creates a three-dimensional network.