Geismar is a municipality in the administrative community of Ershausen/Geismar in the Thuringian district of Eichsfeld. It lies on the border with Hesse. The municipality is located in southern Eichsfeld at the confluence of the Frieda, Rosoppe and Rode rivers. The district town of Heilbad Heiligenstadt is around 15 kilometers to the north and the town of Eschwege 9 kilometers to the southwest.