A new carboxylato-bridged Co-II network of formula {Co((kappa(1)-kappa(1))-(kappa(1)-mu(2))-mu(4)-TDC)(mu(2)-H2O)(0.5)(H2O) }(n) (H2TDC = 2,5-thiophenedicarboxylic acid) has been hydrothermally synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and IR and UV-visible spectroscopies. The title compound is made of chains of Co-II dimers interconnected by TDC2- ligands, showing an unprecedented asymmetric tetradentate coordination mode of the carboxylate functions. Magnetic measurements show weak ferromagnetic interactions between the Co ions within the dimers.