The Battle of Drøbak Sound took place in Drøbak Sound, the northernmost part of the outer Oslofjord in southern Norway, on 9 April 1940. It marked the end of the "Phoney War" and the beginning of World War II in Western Europe.

The Oslofjord is an inlet in southeastern Norway. The 120-kilometre (75 mi) fjord begins at the small village of Bonn in Frogn Municipality and stretching northwards to the city of Oslo, and then curving to the east and then south again. It then flows south to an imaginary line running between the Torbjørnskjær Lighthouse and Færder Lighthouse where it becomes part of the Skagerrak strait. The Skagerrak connects the North Sea and the Kattegat sea area, which leads to the Baltic Sea. The Oslofjord is not a fjord in the geological sense, but in the Norwegian language, the term fjord can refer to a wide range of waterways including inlets such as this one.