Lanthanum(III) sulfide is a binary inorganic chemical compound of lanthanum metal and sulfur with the chemical formula La2S3.

[2][3] Lanthanum(III) sulfide can be prepared from the effect of sulfur vapor on metallic lanthanum: It can also be prepared from the effect of hydrogen sulfide on lanthanum: The effect of hydrogen sulfide on lanthanum oxide in the presence of reducing agents also produces lanthanum(III) sulfide: Dilanthanum trisulfide forms reddish-yellow crystals of the cubic system, with space group I43d, and unit cell parameters a = 0.8706 nm.

[4] It does not dissolve in cold water.

La2S3 is used as a precursor to prepare complex compounds.

It is also used in glass manufacturing and in optical fibers.