The palladium(II)-containing 40-tungsto-4-arsenate(III) [Pd2Na2KAs4W40O140(H2O)]21– was synthesized by reaction of PdSO4 with the cryptate heteropolyanion [As4W40O140]28– in potassium acetate buffer. The title compound was characterized by single-crystal XRD, IR, TGA, and elemental analysis.