In the mol-ecule of the title compound, C(10)H(9)NO(6)S·H(2)O, the benzothia-zine ring adopts an envelope conformation. An intra-molecular N-H⋯O hydrogen bond results in the formation of a nonplanar five-membered ring which has a twisted conformation. In the crystal structure, inter-molecular N-H⋯O, O-H⋯O and C-H⋯O hydrogen bonds link the mol-ecules to form a three-dimensional network. There is a π-π contact between the benzene rings [centroid-centroid distance = 3.972 (2) Å].