There are two independent mol-ecules in the asymmetric unit of the title compound, C13H13NO5S, in both of which the ester substituent is nearly coplanar [C-C-C-O torsion angles = 2.7 (7) and -0.8 (7)°] with the planar fragment of the bicycle due to the formation of a strong O-H⋯O intra-molecular hydrogen bond. The vinyl group at the ring N atom is approximately orthogonal to the heterocyclic mean plane [C-N-C-C torsion angles = 103.1 (6) and 98.2 (5)°]. The refinement was performed on a two-component, non-merohedrally twinned crystal [population ratio = 0.483 (3):0.517 (3).