The Duchy of Holstein was the northernmost state of the Holy Roman Empire, located in the present German state of Schleswig-Holstein. It originated when King Christian I of Denmark had his County of Holstein-Rendsburg elevated to a duchy by Emperor Frederick III in 1474. Members of the Danish House of Oldenburg ruled Holstein – jointly with the Duchy of Schleswig – for its entire existence.

Schleswig-Holstein is the northernmost of the 16 states of Germany, comprising most of the historical Duchy of Holstein and the southern part of the former Duchy of Schleswig. Its capital city is Kiel; other notable cities are Lübeck and Flensburg. It covers an area of 15,763 km2 (6,086 sq mi), making it the 5th smallest German federal state by area.