Nassau-Usingen was a county of the Holy Roman Empire in the Upper Rhenish Circle that became a principality in 1688. The origin of the county lies in the medieval county of Weilnau that was acquired by the counts of Nassau-Weilburg in 1602.That county was divided in 1629 into the lines of Nassau-Weilburg, and Nassau-Saarbrücken that was divided only 30 years later in 1659. The title has been carried in pretense by Prince Frederick August's half-brother Karl Philip's line.