Langenau Castle (German: Schloss Langenau) is an old lowland castle in the municipality of Obernhof in the county of Rhein-Lahn-Kreis in the German state of Rhineland-Palatinate. The site of the fortification was the confluence of the Gelbach and the Lahn rivers. As a result the castle is designed as a lowland type, which is unusual for this region. In 1847, Countess Giech, a daughter of Prussian reform minister, vom und zum Stein, became the new occupant. She had the schloss converted in 1851 into a hospital and home for children of the poor.