Princess Helena Adelaide of Schleswig-Holstein-Sonderburg-Glücksburg was the third eldest daughter of Friedrich Ferdinand, Duke of Schleswig-Holstein and his wife Princess Karoline Mathilde of Schleswig-Holstein-Sonderburg-Augustenburg. She was a princess of Denmark through her marriage within the House of Schleswig-Holstein-Sonderburg-Glücksburg to Prince Harald of Denmark. Princess Helena was a Nazi sympathiser during World War II and was after the war exiled from Denmark, but eventually allowed to return, where she died.