Terbium(III) oxide, also known as terbium sesquioxide, is a sesquioxide of the rare earth metal terbium, having chemical formula Tb2O3.

It is a p-type semiconductor, which conducts protons, which is enhanced when doped with calcium.

[3] It may be prepared by the reduction of Tb4O7 in hydrogen at 1300 °C for 24 hours.

[4] It is a basic oxide and easily dissolved to dilute acids, and then almost colourless terbium salt is formed.

The crystal structure is cubic and the lattice constant is a = 1057 pm.