Lake Untersee is the largest ice-covered freshwater lake in the interior of East Antarctica. The mountain oasis is situated around it in the Gruber Mts. of the Wohlthat Massif. For approximately 7,000 years the area has been free of ice and the local climate relatively stable. It is very severe, cold, and windy and dominated by intense evaporation and sublimation but with little melt. Relative humidity averages only 37%. Vegetation is sparse in the oasis and previously only poorly investigated. Two lichen species and no bryophytes were known from the area. In November-December 2018, a survey of terrestrial flora and vegetation was made. The list of lichens was completed for the area, bryophytes were found for the first time, and some terrestrial algae were collected. In total, 23 lichen species, 1 lichenicolous fungus, 1 moss, and 18 terrestrial algae were discovered for the locality. The abundance of each species within their habitats was also evaluated. The lichen flora of the Untersee Oasis is typical for continental oases and similar to other previously investigated internal territories of Dronning Maud Land, except for the very rich lichen flora of the Schirmacher Oasis.