Tetroncium magellanicum (Juncaginaceae) was described by Willdenow in 1808, based on material collected by Commerson at the Strait of Magellan during Bougainville's voyage around the world. Type material of this species was traced and a lectotype for the name is designated. A description of the species and notes on its ecology and conservation status are provided. For the first time, a detailed map showing the known distribution area of T. magellanicum is presented.