The history of Schleswig-Holstein consists of the corpus of facts since the pre-history times until the modern establishing of the Schleswig-Holstein state.

The Duchy of Schleswig was a duchy in Southern Jutland covering the area between about 60 km north and 70 km (45 mi) south of the current border between Germany and Denmark. The territory has been divided between the two countries since 1920, with Northern Schleswig in Denmark and Southern Schleswig in Germany. The region is also called Sleswick in English.