In the title compound, C15H13N3O3S, the C-S-N(H)-N linkage is non-planar, the torsion angle being -65.12 (13)° and the S atom showing a tetrahedral environment. The compound has two almost planar fragments linked to the S atom: the isatin-derivative fragment [(C8H5NO)N-N(H)-] and the tolyl fragment [C7H7-] have maximum deviations from the mean plane through the non-H atoms of 0.0813 (13) and 0.0094 (16) Å, respectively, and make an interplanar angle of 80.48 (3)°. In the crystal, molecules are connected into inversion dimers via pairs of N-HO hydrogen bonds. Additionally, the molecular structure is stabilized by an intramolecular N-HO hydrogen bond.