Potassium scandium niobate hydroxide, K(3)(Sc(0.875)Nb(0.125))Nb(2)O(9)H(1.75), is a new scandium niobate with a unique cage structure. The structure contains two non-equivalent K(+) sites (3m and 6m2 site symmetry), one disordered Sc(3+)/Nb(5+) site (3m site symmetry), one Nb(5+) site (3m site symmetry), two O(2-) sites (m and mm2 site symmetry) and one H(+) site (m site symmetry). Both scandium and niobium have octahedral environments, which combine to form cages around potassium. One K atom lies in a cube-like cage built of seven octahedra, while the other K atom is encapsulated by an eight-membered trigonal face-bicapped prism. The cages form sheets that extend along the ab plane.