In the zwitterionic title compound, C(9)H(13)NO(2)S, the pyrrolidine heterocycle adopts an envelope conformation (with the C atom in the 7-position as the flap). The negative charge is delocalized over the two carbonyl groups and the C atom connecting them. The positive charge is located on the S atom. Two inter-molecular C-H⋯O inter-actions are observed. The molecular geometry at the S atom is trigonal pyramidal.