The title complex, {(C(2)H(8)N)[Y(C(2)O(4))(2)(H(2)O)]·3H(2)O}(n), was obtained accidentally under hydro-thermal conditions. The Y(III) atom is chelated by four oxalate ligands and one water mol-ecule resulting in a distorted tricapped trigonal-prismatic geometry. Each oxalate ligand bridges two Y(III) atoms, thus generating a three-dimensional network with cavities in which the ammonium cations and lattice water mol-ecules reside. Various O-H⋯O and N-H⋯O hydrogen-bonding inter-actions stabilize the crystal structure. The title complex is isotypic with the Eu and Dy analogues.