Nassau-Siegen was a principality within the Holy Roman Empire that existed between 1303 and 1328, and again from 1606 to 1743. From 1626 to 1734, it was subdivided into Catholic and Protestant parts. Its capital was the city of Siegen, founded in 1224 and initially a condominium jointly owned by the archbishopric of Cologne and Nassau. It was located some 50 km east of Cologne, and it contained the modern localities of Freudenberg, Hilchenbach, Kreuztal, Siegen, and Wilnsdorf.

The County of Nassau was a German state within the Holy Roman Empire and later part of the German Confederation. Its ruling dynasty, the male line of which is now extinct, was the House of Nassau.