The Prussian province of Hesse-Nassau (German Hessen-Nassau) was created in 1868 as a consequence of the Austro-Prussian War of 1866 by combining the previously independent Electorate of Hesse (Hesse-Kassel), the Duchy of Nassau, the Free Imperial City of Frankfurt, areas gained from the Kingdom of Bavaria, and areas gained from the Grand Duchy of Hesse (Hesse-Darmstadt; including part of the former Landgraviate of Hesse-Homburg). The capital of Hesse-Nassau was Cassel; it was subdivided into two administrative regions: Cassel and Wiesbaden .

In 1929, the Republic of Waldeck became a part of Hesse-Nassau. In 1944, Hesse-Nassau was split into the provinces of Electoral Hesse (German Kurhessen) with Kassel as its capital and Nassau with Wiesbaden as its capital. In 1945, after the end of World War II, they were merged with the neighbouring Republic of Hesse (Hesse-Darmstadt) into the newly founded State of Hesse. Parts of Nassau were moved to the State of Rhineland-Palatinate.