In the cation of the title compound, C11H10N3+·Br−·H2O, the central 1,3,5-triazinium ring has a flattened boat conformation and the outer heterocyclic rings are nearly planar [maximum deviations = 0.010 (4) and 0.009 (4) Å] and twisted with respect to each other with a dihedral angle of 22.87 (19)°. In the crystal, classical O—H...Br hydrogen bonds and weak C—H...Br and C—H...O hydrogen bonds link the cations, bromide anions and water molecules of crystallization into a three-dimensional supramolecular network.