Thulium(III) oxide is a pale green crystalline compound, with the formula Tm2O3.

It was first isolated in 1879, from an impure sample of erbia, by Swedish chemist Per Teodor Cleve, who named it thulia.

One method involves burning thulium metal or its various salts in air.

[1] Thulium(III) oxide (Tm₂O₃) is a pale green, thermally stable powder with a high melting point of 2,341 °C and a density of 8.6 g/cm³, typically forming a cubic crystal structure.

[5] Due to its unique f-electron configuration, Tm₂O₃ has notable optical properties.