Prussian blue, a blue pigment, belongs to the ferric cyanoferrate (II) group. It binds univalent metal ions, and the binding activity depends on their ionic radius. Prussian blue is not absorbed from the gastrointestinal tract in a significant amount. The cesium is entero-enteric cycled through the intestine. Prussian blue inhibits the reabsorption of the cesium. Many studies using experimental animals showed that the oral administration of Prussian blue increases the rate of the fecal excretion of radiocesium which results in shortening its biological half life. The accident in Goiania, Brazil, in 1987 showed that Prussian blue effectively accelerated the removal of radiocesium without toxicity. In the present review, we describe Prussian blue from a biological aspect and discuss its clinical application for the decontamination of cesium in radiation accidents.