The Duchy of Nassau (German: Herzogtum Nassau), or simply Nassau, was an independent state between 1806 and 1866, located in what is now the German states of Rhineland-Palatinate and Hesse. It was a member of the Confederation of the Rhine and later of the German Confederation. Its ruling dynasty, now extinct, was the House of Nassau. The duchy was named for its historical core city, Nassau, although Nassau was not its capital. As of 1865, it had 465,636 inhabitants. After being occupied and annexed into the Kingdom of Prussia in 1866, it was succeeded by the Province of Hesse-Nassau. The area today is a geographical and historical region, Nassau, and Nassau is also the name of the Nassau Nature Park within the borders of the former duchy.
Today, the Grand Duke of Luxembourg still uses "Duke of Nassau" as his secondary title (of pretense), and "Prince" or "Princess of Nassau" is used as a title of pretense by other members of the grand ducal family. Nassau is also part of the name of the Dutch Royal Family, which styles itself Orange-Nassau.