In the title compound, C(11)H(11)NO(4)S, the thia-zine ring adopts a distorted half-chair conformation. The enolic H atom is involved in an intra-molecular O-H⋯O hydrogen bond, forming a six-membered ring. Mol-ecules are linked through weak inter-molecular C-H⋯O hydrogen bonds, resulting in chains lying along the b axis.