In the title compound, [KLu(C(2)O(4))(2)(H(2)O)(4)](n), the Lu(III) ion lies on a site of [Formula: see text] symmetry in a dodeca-hedron defined by eight O atoms from four oxalate ligands. The K atom lies on another site of the same symmetry and is coordinated by four oxalate O atoms and four O water atoms. The mid-point of the C-C bond of the oxalate group lies on an inversion center. In the packing structure, each oxalate ligand links two Lu(III) and two K atoms, forming a three-dimensional open framework with channels running along [001]. Inter-molecular O-H⋯O hydrogen bonds occur.