Sematophyllum lebouvieri Ochyra is described as a new species from Îles Kerguelen in the Subantarctic and Isla de los Estados in the Tierra del Fuego archipelago in southern South America. The new species is an aquatic moss, closely related to S. secundifolium (Müll. Hal.) Mitt. from Isla Hermite near Cape Horn, but it is distinct at a glance in being a robust plant with larger, broadly ovate or elliptical, not secund leaves which are rounded or obtuse-apiculate to subacute at the apex and have plane to narrowly reflexed margins that are distinctly serrulate at the apex. The discovery of this species on Îles Kerguelen represents a new addition of the genus Sematophyllum and the family Sematophyllaceae to the bryoflora of the Subantarctic.