Terbium(III) oxalate is the oxalate of terbium with the chemical formula Tb<sub>2</sub>(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>. Its decahydrate can be obtained by reacting terbium(III) chloride and oxalic acid in an aqueous solution. Its decahydrate gradually loses water when heated and becomes anhydrous. Continued heating obtains terbium(III,IV) oxide. It decomposes in isolation from air to form terbium(III) oxide. The decomposed gas products are carbon monoxide and carbon dioxide. It reacts with hydrochloric acid to obtain H[Tb(C<sub>2</sub>O<sub>4</sub>)<sub>2</sub>]÷6H<sub>2</sub>O.