Crystals of the title compound are monoclinic, a= 8·729(2), b= 14·285(6), c= 11·622(4)Å, β= 102·66(2)°, Z= 8, space group P2₁/c. The structure was determined from X-ray diffractometer data by Patterson and Fourier syntheses, and refined by full-matrix least-squares calculations to R 0·045 for 2401 observed reflections. The structure contains two cadmium atoms and two maleate ligands, both pairs of which are crystallographically and chemically distinct. One cadmium atom is six-co-ordinate through four water molecules and two bridging maleate oxygen atoms while the other is eight-co-ordinate, in distorted dodecahedral geometry, through four chelated carboxy-groups from the two maleate ligands. The maleate ligands link cadmium atoms into a three-dimensional polymer which is further strengthened by extensive hydrogen bonding through the water molecules. Cd–O distances are 2·199(5)–2·843(5)Å, while the bridging Cd–O bonds [2·289(4) and 2·285(4)Å] suggest relatively high interaction. The structure exhibits chelation and bridging by two separate carboxy-groups, a mode of co-ordination uncommon in carboxylates.