We have synthesized and structurally characterized the 4-palladium(II)-containing 30-tungsto-4-arsenate(V), [Pd-4(As2W15O56)(2)](16-) (1), which represents the first palladium(II)-containing tungstoarsenate(V). The title polyanion 1 was prepared by a simple one-pot procedure in aqueous medium and characterized by single-crystal X-ray diffraction (XRD), thermogravimetric analysis (TGA), cyclic voltammetry, elemental analysis, and W-183 nuclear magnetic resonance (NMR), infrared (IR), and ultraviolet-visible (UV-vis) spectroscopies. Polyanion 1 consists of four Pd2+ ions that are coordinated in a square-planar geometry to two trilacunary [As2W15O56](12-) Wells-Dawson fragments resulting in a sandwich-type assembly. Catalytic studies on 1 revealed that it is an efficient catalyst precursor for the Suzuki-Miyaura cross-coupling reactions of various aryl halides in aqueous and nonaqueous media.