In the title mixed-ligand metal–organic polymeric compound, {[Cd(C14H8O6S)(C16H16N2)]·3H2O}n, the asymmetric unit contains a crystallographically unique CdII atom, one doubly deprotonated 4,4′-sulfonyldibenzoic acid (H2SDBA) ligand, one 3,4,7,8-tetramethyl-1,10-phenanthroline (TMPHEN) molecule and three solvent water molecules. Each CdII centre is six-coordinated by two O atoms from a chelating carboxylate group of a SDBA2− ligand, two O atoms from monodentate carboxylate groups of two different SDBA2− ligands and two N atoms from a chelating TMPHEN ligand. There are two coordination patterns for the carboxylate groups of the SDBA2− ligand, with one in a μ111 chelating mode and the other in a μ211 bis-monodentate mode. Single-crystal X-ray diffraction analysis revealed that the title compound is a one-dimensional double-chain polymer containing 28-membered rings based on the [Cd2(CO2)2] rhomboid subunit. More interestingly, a chair-shaped water hexamer cluster is observed in the compound.