The title compound, C(7)H(8)O(4), isolated from Opuntia dillenii Haw (Cacta-ceae), is almost planar [maximum deviation of 0.027 (2) Å] except for the H atoms of the methylene and methyl groups. The crystal packing is stabilized by C-H⋯O and O-H⋯O inter-molecular hydrogen bonds, resulting in the formation of a three-dimensional network.