In the title compound, C(16)H(15)N(3)O(3)S, the dihedral angle between the aromatic rings is 8.18 (11)° and the C=N-N=C torsion angle is 178.59 (14)°. The conformation of the thia-zine ring is an envelope, with the S atom displaced by 0.8157 (18) Å from the mean plane of the other five atoms (r.m.s. deviation = 0.045 Å). An intra-molecular O-H⋯N hydrogen bond closes an S(6) ring. In the crystal, weak C-H⋯O inter-actions link the mol-ecules, with all three O atoms acting as acceptors.