A new species of Uncinia (Cyperaceae), U. austroamericana, is described and illustrated from austral South America and from the Tristan da Cunha archipelago in the south-Atlantic Ocean. This species grows in persistently wet, base-poor sites, particularly in Sphagnum bogs, and is known from southern Patagonia, Tierra del Fuego, and from the largest island of the Tristan da Cunha group. It differs both morphologically and ecologically from the similar-appearing U. macrolepis, a minerotrophic species best known from moist depressions in grasslands. The new species differs morphologically from U. macrolepis by possessing achenes that, when mature, are pale-colored and conspicuously-thickened at the apex and, also, by having more or less loosely-flowered spikes that frequently exceed 2 cm in length. Additionally, U. chilensis is reported for the first time from Argentina.