The title polymeric compound, {[Cd2(C7H3NO5)2(H2O)4]·4H2O}n or {[Cd2(hypydc)2(H2O)4]·4H2O}n (where hypydcH2 is 4-hydroxy­pyridine-2,6-dicarboxylic acid), was synthesized by the reaction of cadmium(II) nitrate hexa­hydrate with 4-hydroxy­pyridine-2,6-dicarboxylic acid and propane-1,3-diamine, in a 1:2:2 molar ratio in aqueous solution. The compound is a seven-coordinate binuclear polymeric complex with distorted penta­gonal bipyramidal geometry around CdII [Cd—O = 2.247 (4)–2.474 (3) Å]. In the binuclear monomeric units, the central atoms join together by O atoms of two bridging tridentate (hypydc)2− ligands, and the polymer propagates via two bridging water mol­ecules that link each CdII centre of one monomer to the adjacent neighbour. Propane-1,3-diamine (pn) does not appear in the product but plays a role as a base. Inter­molecular O—H⋯O and C—H⋯O hydrogen bonds, and π–π stacking inter­actions, with distances of 3.725 (3) and 3.766 (3) Å, connect the various components.