In the mol-ecule of the zwitterionic title compound, C(10)H(9)NO(3)S, an intra-molecular N-H⋯O hydrogen bond results in the formation of an almost planar six-membered ring (r.m.s daviation = 0.0150 Å), which is oriented at a dihedral angle of 1.63 (3)° with respect to the naphthalene ring system. In the crystal structure, inter-molecular N-H⋯O hydrogen bonds link the mol-ecules into a two-dimensional network.