The Inner German border (German: innerdeutsche Grenze or deutsch–deutsche Grenze; initially also Zonengrenze, "Zonal boundary") was the border between the German Democratic Republic (GDR or East Germany) and the Federal Republic of Germany (FRG or West Germany) from 1949 to 1990. It ran north-south in a 1,381 kilometres (858 mi) long, irregular L-shaped line from Dassow at the Baltic Sea to Eichigt at the border with Czechoslovakia. The better-known Berlin Wall was a physically separate, less elaborate, and much shorter border barrier surrounding West Berlin, more than 170 kilometres (110 mi) to the east of the Inner German border.

The Inner German border was formally established by the Potsdam Agreement on 1 August 1945 as the boundary between the Western and Soviet occupation zones in Allied-occupied Germany. On the Eastern side, it was made one of the world's most heavily fortified frontiers, defined by a continuous line of high metal fences and walls, barbed wire, alarms, anti-vehicle ditches, watchtowers, automatic booby traps and minefields. It was patrolled by 50,000 armed Border Troops of the GDR who faced tens of thousands of West German, British and US guards and soldiers. In the hinterlands behind the border, more than a million NATO and Warsaw Pact troops awaited the possible outbreak of World War III in Europe. It was a physical manifestation of Winston Churchill's metaphorical Iron Curtain that separated the Soviet and Western blocs during the Cold War. Built by the East German government in phases from 1952 to the late 1980s, the fortifications were constructed to stop Republikflucht, the large-scale emigration of East German citizens to the West, about 1,000 of whom are said to have died trying to cross it during its 45-year existence. It caused widespread economic and social disruption on both sides; East Germans living nearby suffered especially draconian restrictions.

On 9 November 1989, during Die Wende, the East German government announced the opening of the Inner German border and the Berlin Wall. Subsequently, millions of East Germans poured into the West to visit, and hundreds of thousands moved permanently to the West in the following months. More crossings were opened, and ties between long-divided communities were re-established as border controls became little more than a cursory formality. The Inner German border was not completely abandoned until 1 July 1990, exactly 45 years to the day since its establishment, and only three months before German reunification formally ended the GDR. Little remains of its fortifications, and its route has been declared part of a European Green Belt linking national parks and nature reserves along the course of the old Iron Curtain from the Arctic Circle to the Black Sea. Museums and memorials along the old border commemorate the division and the reunification of Germany and, in some places, preserve elements of the fortifications.

The inner German border originated from the Second World War Allies' plans to divide a defeated Germany into occupation zones. The boundaries between these zones were drawn along the territorial boundaries of 19th-century German states and provinces that had largely disappeared with the unification of Germany in 1871. Three zones were agreed on, each covering roughly a third of Germany's territories: a British zone in the north-west, an American zone in the south and a Soviet zone in the east. France was later given a zone in the far west of Germany, carved out of the British and American zones.

The division of Germany was official on 1 August 1945. Because of the unexpectedly rapid Allied advances through central Germany in the final weeks of the war, British and American troops occupied large areas of territory that had been assigned to the Soviet zone of occupation. The redeployment of Western troops prompted many Germans to flee west to escape the Russians' takeover of the remainder of the Soviet zone.

The wartime Allies initially worked together under the auspices of the Allied Control Council (ACC) for Germany. Cooperation between the Western Allies and the Soviets ultimately broke down because of disagreements over Germany's political and economic future. In May 1949, the three western occupation zones were merged to form the Federal Republic of Germany (FRG), a capitalist state with free and fair elections. The Soviet zone became the German Democratic Republic (GDR), a communist state where voters were restricted to electing communist candidates.

From the outset, West Germany and the western Allies rejected East Germany's legitimacy. The creation of East Germany was deemed a communist fait accompli, without a freely or fairly elected government. West Germany regarded German citizenship and rights as applying equally to East and West German citizens. An East German who escaped or was released to the West was automatically granted West German rights, including residence and the right to work; West German laws were deemed to be applicable in the East. East Germans thus had a powerful incentive to move to the West, where they would enjoy greater freedom and economic prospects. The East German government sought to define the country as a legitimate state in its own right and portrayed West Germany as enemy territory (feindliches Ausland) – a capitalist, semi-fascist state that exploited its citizens, sought to regain the lost territories of the Third Reich, and stood opposed to the peaceful socialism of the GDR.

In the early days of the occupation, the Allies controlled traffic between the zones to manage the flow of refugees and prevent the escape of former Nazi officials and intelligence officers. These controls were gradually lifted in the Western zones, but were tightened between Western and Soviet zones in 1946 to stem a flow of economic and political refugees from the Soviet zone. Between October 1945 and June 1946, 1.6 million Germans left the Soviet zone for the west.