In the zwitterionic title compound, C18H16N2O4·H2O, the dihedral angle between the planes of the benzene and indole rings is 39.20 (8)°. An intramolecular N—H⋯O hydrogen bond generates an S(6) ring motif. In the crystal, intermolecular hydroxy and water O—H⋯O(carboxylate) and N+—H⋯O(carboxylate) and indole N—H⋯O(water) hydrogen bonds give a three-dimensional structure