In the title compound, C13H10O4·H2O, both the carboxylic acid [Car-Car-C-O = -121.1 (2)°, where ar = aromatic] and the ester [Car-Car-O-C = -104.4 (3)°] groups lie out of the mean plane of the conjugated aromatic system. In the crystal, the organic mol-ecule is hydrogen bonded to water mol-ecules through the ester and carb-oxy moieties, forming chains along the a-axis direction. The methyl H atoms of the acet-oxy group are disordered over two equally occupied sites.