In K(4)Sn(9), which crystallizes with a new structure type, the Sn atoms form isolated Wade nido-[Sn(9)]4- clusters of approximate C(4v) symmetry (monocapped square antiprisms), with Sn-Sn distances ranging from 2.9264 (9) to 3.348 (1) A. The cluster anions are separated by K+ cations and are in a hexagonal close-packed arrangement.