Arnstadt is a town in Ilm-Kreis, Thuringia, Germany, on the river Gera about 20 kilometres south of Erfurt, the capital of Thuringia. Arnstadt is one of the oldest towns in Thuringia, and has a well-preserved historic centre with a partially preserved town wall. The town is nicknamed Das Tor zum Thüringer Wald because of its location on the northern edge of that forest. Arnstadt has a population of some 27,000. The city centre is on the west side of Gera. The municipality has absorbed several neighbouring municipalities: Angelhausen–Oberndorf (1922), Siegelbach (1994), Rudisleben (1999) and Wipfratal (2019). The neighbouring municipalities are Amt Wachsenburg, Alkersleben, Dornheim, Bösleben-Wüllersleben, Stadtilm, Ilmenau, Plaue and Geratal.