The title compound, C(17)H(20)N(2)O(3)S, synthesised from sulfanilic acid and 4-diethyl-amino-benzaldehyde, crystallized out as a zwitterion with the central N atom protonated. The zwitterion exists in an E conformation with respect to the C=N double bond. The dihedral angle between the benzene rings is 37.57 (5)°. In the crystal, the zwitterions are linked into a tape along the a axis by N-H⋯O hydrogen bonds. The crystal structure is further stabilized by weak C-H⋯O inter-actions and π-π inter-actions with a centroid-centroid distance of 3.8541 (6) Å. An O⋯O [2.8498 (11) Å] short contact is present.