A new carboxylato-bridged CoII network of formula {Co((kappa1-kappa1)-(kappa1-mu2)-mu4-TDC)(mu2-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 CoII 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.