The title compound, C(11)H(9)N(3)O(4)S·H(2)O, contains both an acid and a base centre, and displays a zwitterionic structure in the solid state. The benzene ring makes an angle of 109.1 (2)° with the pyridinium ring. The crystal structure is stabilized by O-H⋯N, O-H⋯O and N-H⋯O hydrogen bonds.