The title compound, C7H6O5 center dot H2O, is a new polymorph of the structures reported by Jiang et al. (2000) [Acta Cryst. C56, 594-595] and Okabe et al. (2001) [Acta Cryst. E57, o764-o766]. The gallic acid molecule is essentially planar (r.m.s. deviation = 0.550 A). An intramolecular O-H...O hydrogen bond occurs in the gallic acid molecule, which is linked to the water molecule by a further O-H...O hydrogen bond. In the crystal, the components are linked by O-H...O hydrogen bonds. The hydrogen-bonding pattern differs from those reported for the previous polymorphs.