It takes its name from the Schlei (Slien), an inlet of the Baltic Sea at the end of which it sits, and vik or vig which means "bay" in Old Norse and Danish.

The Viking settlement of Hedeby, located south of the modern town, was first mentioned in 804.

In 1050, following several destructions, the population was moved to the opposite shore of the Schlei, becoming the city of Schleswig.

In 1066 Hedeby was finally destroyed, and Schleswig remained a part of the Danish kingdom.

The climate is oceanic (Köppen: Cfb), humid and mild with a slight continental influence.