The title compound, C13H9NO4, crystallizes in a zwitterionic form with the pyridine N atom protonated and the carboxyl OH group deprotonated. The benzene and pyridinium rings are inclined with a dihedral angle of 31.42 (14)° between them. A previous report of this stucture claims, we believe incorrectly, that neither of the carboxylate groups is deprotonated [Zhang et al. (2010 ▶). Acta Cryst. E66, o2928–o2928]. In the crystal, intermolecular O—H⋯O, N—H⋯O and weak C—H⋯O hydrogen-bonding interactions link adjacent molecules into a three-dimensional supramolecular network.