Radium sulfate (or radium sulphate) is an inorganic compound with the formula RaSO<sub>4</sub> and an average molecular mass of 322.088 g/mol. This white salt is the least soluble of all known sulfate salts. It was formerly used in radiotherapy and smoke detectors, but this has been phased out in favor of less hazardous alternatives.
Radium sulfate crystallizes in a solid in the same structure as barium sulfate. It forms crystals in the orthorhombic crystal system, with a unit cell of dimensions a = 9.13 b=5.54 and c = 7.31 à. The unit cell volume is 369.7 à<sup>3</sup>. Distance from the radium ion to oxygen is 2.96 âÂÂàand the sulfur to oxygen bond length in the sulfate ion is 1.485 âÂÂà. In this compound the ionic radius of the radium ion is 1.66 à, and it is in ten coordination.
Radium sulfate can form solid solutions with the sulfates of strontium, barium or lead.