Rheinfels Castle (German: Burg Rheinfels) is a castle ruin located in Sankt Goar, Germany overlooking the Rhine. It was started in 1245 by Count Diether V of Katzenelnbogen and was slighted by French Revolutionary Army troops in 1797. It is the largest castle overlooking the Rhine, and historically covered five times its current area.

While much of the castle is a ruin, some of the outer buildings are now a luxury hotel, "wellness" centre and restaurant.[citation needed] There is also a museum within some of the better preserved structures.