Promethium(III) hydroxide is an inorganic compound with a chemical formula Pm(OH)<sub>3</sub>. It is a radioactive compound.
Promethium(III) hydroxide can be produced by reacting ammonia water with a solution of a promethium(III) salt, from which promethium(III) hydroxide precipitates as a purplish-pink amorphous solid. By heating with water, it can be forced to crystallize, giving hexagonal crystals isomorphous with other rare earth hydroxides: