In the title compound, C16H11NO4S, the nine-membered fused ring is nearly planar, with maximum deviations from the mean plane of −0.022 (1) Å for the N atom and 0.011 (1) Å for the S atom, and makes a dihedral angle of 53.56 (7)° with the phenyl ring. The crystal structure is stabilized by O—H...O and C—H...O hydrogen-bonding interactions.