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. There are two independent mol-ecules and two water mol-ecules in the asymmetric unit. The dihedral angles between the benzene ring and the pyridinium ring are 109.7 (1) and 110.7 (1)°. The dihedral angles between the nitro group and the benzene ring are 116.1 (2) and 116.7 (1)°. The crystal structure is stabilized by N-H⋯O, O-H⋯N and O-H⋯O hydrogen bonds.