Anhalt-Köthen [ˈʔanhalt ˈkøːtn] was a principality of the Holy Roman Empire ruled by the House of Ascania. It was created in 1396 when the Principality of Anhalt-Zerbst was partitioned between Anhalt-Dessau and Anhalt-Köthen. The first creation lasted until 1562, when it fell to Prince Joachim Ernest of Anhalt-Zerbst, who merged it into the reunited Principality of Anhalt.
Anhalt-Köthen was created a second time in 1603, when Anhalt was again divided. In 1806, Anhalt-Köthen was raised to a duchy. With the death of Duke Henry on 23 November 1847, the Anhalt-Köthen line became extinct and its territories were united to Anhalt-Dessau by patent of 22 May 1853. Today, Anhalt-Köthen is mostly remembered as a long-time residence of Johann Sebastian Bach, while he worked for Leopold, Prince of Anhalt-Köthen.