The title compound, K-4[(MoO2)-O-VI(SO4)(3)], was precipitated from a melt of molybdenum(VI) oxide and potassium sulfate in potassium disulfate. The compound contains monomeric [(MoO2)-O-VI(SO4)(3)](4-) anions, with the Mo-VI atom, both oxide ligands, and the S atom and both ligating O atoms of the bidentate sulfate group lying on a crystallographic mirror plane. One of the potassium cations is nine-coordinate, while the other is eight-coordinate.