A new endemic species of Sagina L., Sagina hookeri Timaná, sp. nov. (Caryophyllaceae) is described for the flora of Île Amsterdam, in the southern Indian Ocean (French Southern and Antarctic Lands). Differences between this taxon and S. diffusa (Hook.f.) Timaná (endemic to the neighboring island of Saint-Paul) and S. procumbens L. (introduced in several circum-austral islands) are discussed, and an identification key to those three Sagina species occurring in Saint-Paul and Amsterdam islands is presented.