The Duke of Nassau was a title in the German nobility, associated with the region of Nassau in present-day Germany. The title was established in the 19th century and was held by members of the House of Nassau, a prominent noble family. The most notable Duke was Adolph, Duke of Nassau, who ruled from 1839 until his death in 1866. The duchy played a significant role in the political landscape of Germany, particularly during the formation of the German Empire in 1871. The title eventually became obsolete after the abolition of the monarchy in Germany following World War I, leading to the dissolution of noble titles.