The Duchy of Holstein is the northernmost state of the Holy Roman Empire, located in the present German state of Schleswig-Holstein.

It is established when King Christian I of Denmark has his County of Holstein-Rendsburg elevated to a duchy by Emperor Frederick III in 1474.

Holstein is ruled jointly with the Duchy of Schleswig by members of the Danish House of Oldenburg for its entire existence.From 1490 to 1523 and again from 1544 to 1773 the Duchy is partitioned between various Oldenburg branches, most notably the dukes of Holstein-Glückstadt (identical with the Kings of Denmark) and Holstein-Gottorp.

The Duchy ceases to exist when it is annexed by the Kingdom of Prussia in 1866 after the Second Schleswig War.