Arnstadt is a town in Ilm-Kreis, Thuringia, Germany, situated on the Gera River. It is one of the oldest towns in Thuringia and is nicknamed Das Tor zum Thüringer Wald, The Gate to the Thuringian Forest.

Modern Arnstadt has a population of approximately 25,000. It is a manufacturing center with glassworks and foundries, a solar panel production plant as well as glove-manufacturing and wood-finishing businesses. Manganese is one of the ores mined in the vicinity.

The town hosts an annual high jump meet – Hochsprung mit Musik – which attracts some of the world's foremost high jumpers. Kajsa Bergqvist set a world record in Arnstadt's Sporthalle am Jahn-sportpark in 2006.

In the 14th century Arnstadt was ruled by the counts of Schwarzburg and later became a capital of Schwarzburg-Sondershausen.

The composer Johann Sebastian Bach, whose family lived for generations in the vicinity of Arnstadt, became the organist at St Boniface's church in 1703. Most Bach scholars believe his well-known "Toccata and Fugue in D Minor" was written here between 1703 and 1709.