The process to determine the current course of the Danish–German border was organized by the Treaty of Versailles following Germany’s loss in World War I. The general delimitation of the boundary was decided in 1920 by referendum in two zones of contested territory, which loosely divided the German and Danish speaking populations. The boundary was formalized the following year by a boundary commission. 

The land boundary between Denmark and Germany separates the Danish peninsula of Jutland from the rest of continental Europe. It runs generally east to west between the Baltic Sea and the North Sea. The total length of the frontier is 146 kilometers (91 miles), and it is divided into three sections. In the center lies a 68 kilometer (42 mile) land boundary between two internal waters frontiers. On the eastern side, a 43 kilometer (27 mile) border extends from the land boundary terminus through the Flensborg Fjord and Gelting Bay to the Baltic Sea. To the west, the boundary is 35 kilometers (22 miles) and terminates at a point in the North Sea between the Danish island of Rømø and the German island of Sylt.

Germany and Denmark have two established maritime boundaries, one in the Baltic Sea and one in the North Sea.