Holstein-Glückstadt or Schleswig-Holstein-Glückstadt is the historiographical name, as well as contemporary shorthand name, for the parts of the duchies of Schleswig and Holstein that were ruled by the Kings of Denmark in their function as dukes of Schleswig and Holstein, thus also known as Royal Schleswig-Holstein.
The main centre of administration was Segeberg and from 1648 Glückstadt (founded in 1617) on the River Elbe.
In 1640, the County of Holstein-Pinneberg, whose ruling house was extinct, was merged into the royal part of the Duchy of Holstein.
In 1713, the estates of the Dukes of Schleswig-Holstein-Gottorp in Schleswig including Schloss Gottorf were conquered by royal Danish troops in the course of the Great Northern War and in the 1720 Treaty of Frederiksborg, Duke Charles Frederick of Schleswig-Holstein-Gottorp had to cede them to his liege lord the Danish crown.
His remaining territories in Holstein formed the Duchy of Holstein-Gottorp administered from Kiel.