Nassau-Usingen was a county of the Holy Roman Empire in the Upper Rhenish Circle that became a principality in 1688.

At the beginning of the 18th century, three of the Nassau lines died out and Nassau-Usingen became their successor (1721 Nassau-Idstein, 1723 Nassau-Ottweiler und 1728 Nassau-Saarbrücken).

On July 17, 1806, the counties of Nassau-Usingen and Nassau-Weilburg joined the Confederation of the Rhine.

Under pressure from Napoleon, both counties merged to become the Duchy of Nassau on August 30, 1806, under joint rule of Prince Frederick August of Nassau-Usingen and his younger cousin Prince Frederick William of Nassau-Weilburg.

The title has been carried in pretense by Prince Frederick August's half-brother Karl Philip's line.