Thulium(III) hydroxide is an inorganic compound with the chemical formula Tm(OH)<sub>3</sub>. It can be prepared by the addition of ammonia solution to a solution containing a soluble thulium salt.
Thulium(III) hydroxide reacts with acids and produces thulium(III) salts:
Thulium(III) hydroxide decomposes to TmO(OH) at elevated temperature. Further decomposition produces Tm<sub>2</sub>O<sub>3</sub>.
Thulium(III) hydroxide reacts to form thulium acetylacetonate in the presence of absolute ethanol and acetylacetone. The product precipitates upon cooling of the solution.