Themis Nunatak () is a very large, flat-topped nunatak lying 6 nautical miles (11 km) west-southwest of Mount Umbriel in the southern portion of Alexander Island, Antarctica. The nunatak was first mapped from trimetrogon air photography taken by Ronne Antarctic Research Expedition in 1947-48, and from survey by Falkland Islands Dependencies Survey in 1948-50. Named by United Kingdom Antarctic Place-Names Committee in association with nearby Saturn Glacier, Themis being one of the satellites of the planet Saturn, the sixth planet of the Solar System.