It emerged on the lower Lahn river in what is today Rhineland-Palatinate. The castle of Nassau was built in 1125, and since 1160 the county was named after the castle.
In 1255 Nassau was divided for the first time. During the next centuries several states named Nassau emerged. One of those counties was Nassau-Dillenburg, from where the House of Orange-Nassau is derived.
Due to the extinction of most lines Nassau became a united duchy again in 1806. In 1866 it was annexed by Prussia in order to become incorporated in the province of Hesse-Nassau.