In the zwitterionic title compound, C(17)H(11)NO(5), the dihedral angle between the two aromatic rings is 76.90 (7)°. The dihedral angles between the carboxyl groups and the benzene ring are 64.02 (9) and 21.67 (9)°, the larger angle being associated with an intra-molecular N-H⋯O(carbox-yl) hydrogen bond, resulting from proton transfer from the carb-oxy-lic acid group to the quinoline N atom and giving an S(9) ring motif. In the crystal, mol-ecules are connected by O-H⋯O hydrogen bonds into chains extending along the b-axis direction. An overall two-dimensional network structure is formed through π-π inter-actions between the quinoline rings [minimum ring-centroid separation = 3.6068 (6) Å].