The title complex, [Er(C(5)H(5)NO)(C(2)O(4))(H(2)O)](n), is a new erbium polymer based on oxalate and 4-oxidopyridinium ligands. The Er(II) center is coordinated by six O atoms from three oxalate ligands, one O atom from a 4-oxidopyridinium ligand and one water mol-ecule, and displays a distorted square-anti-prismatic coordination geometry. The oxalate ligands are both chelating and bridging, and link Er(II) ions, forming Er-oxalate layers in which the attached water and 4-oxidopyridinium units point alternately up and down. A mirror plane passes through the Er atom, one C, the oxide O and two oxalate O atoms. The layers are assembled into a three-dimensional supra-molecular network via inter-molecular hydrogen bonding and π-π stacking inter-actions [centroid-centroid distances of 3.587 (2) Å between parallel pyridinium rings]. Both the water mol-ecule and the 4-oxidopyridinium ligand are disordered over two sites in a 1:1 ratio.