Arnstadt, a town of Germany, in the principality of Schwarzburg-Sondershausen, situated on the Gera and on the northern slope of the Thuringian Forest, 10 m. S. of Erfurt; pop. in 1871, 8,603. Among the most notable buildings is a church built in the 11th century. The town has a castle belonging to the reigning family, and a gymnasium with a considerable library. Arnstadt is one of the oldest towns of Thuringia, being mentioned as early as 704. It was formerly known as an emporium for the trade in timber and fruit, and has in modern times become a seat of manufactures.