Flag of the Duchy of Nassau , an independent country until 1866
Arms of Nassau: Azure, billetty a lion rampant with a closed crown Or, armed and langued gules.
St. Goarshausen , with Katz Castle and the Loreley rock
Nassau ( NASS -aw , also NASS -ow , NAHSS -ow , German: [ˈnasaʊ] ⓘ ) is a geographical, historical and cultural region in today's Rhineland-Palatinate and Hesse in western Germany . Named for the town of Nassau , it includes the territory of the Duchy of Nassau , a former sovereign country which existed until 1866. Occupied by Prussia and annexed into the Province of Hesse-Nassau in 1866, Nassau briefly became the name of a separate province, the Province of Nassau , in 1944. Much of the area is today part of the Nassau Nature Park . Nassau is also the name of the smaller Nassau collective municipality , the area surrounding the town of Nassau, which has been merged into Bad Ems-Nassau collective municipality.