The title compound, C10H7NO2S, provides the first structure of an α-alkenyl oxathiazolone ring. The phenyl ring and the oxathiazolone groups make dihedral angles of 0.3 (3) and −2.8 (3)°, respectively, with the plane of the central alkene group; the dihedral angle between the rings is 2.68 (8)°. A careful consideration of bond lengths provides insight into the electronic structure and reactivity of the title compound. In the crystal, extended π-stacking is observed parallel to thea-axis direction, consisting of cofacial head-to-tail dimeric units [centroid–centroid distance of 3.6191 (11) Å]. These dimeric units are separated by a slightly longer centroid–centroid distance of 3.8383 (12) Å, generating infinite stacks of molecules.