In the title compound, C(7)H(6)O(4)S, a monoester derivative of 2,5-thio-phene-dicarboxylic acid, the carboxylic acid and the carboxylic acid ester groups are approximately coplanar with thio-phene ring, making a dihedral angle of 3.1 (4) and 3.6 (4)°, respectively. In the crystal structure, mol-ecules are connected by classical inter-molecular O-H⋯O hydrogen bonds, forming centrosymmetric dimers.