The Principality of Nassau-Usingen was a small German state that existed from 1806 to 1866. It was part of the larger region of Nassau and was located in what is now Germany. The principality was established when the former County of Nassau was elevated to principality status during the Napoleonic Wars. Nassau-Usingen was ruled by the House of Nassau, specifically the branch known as the Usingen line. In 1866, the principality was dissolved and incorporated into the newly formed Prussian Province of Hesse-Nassau following the Austro-Prussian War.