German reunification (German: Deutsche Wiedervereinigung) was the process in 1990 in which the German Democratic Republic (GDR; German: Deutsche Demokratische Republik, DDR) became part of the Federal Republic of Germany (FRG; German: Bundesrepublik Deutschland, BRD) to form the reunited nation of Germany.

The end of the unification process is officially referred to as German unity (Deutsche Einheit), celebrated each year on 3 October as German Unity Day (Tag der deutschen Einheit). East and West Berlin were reunited into a single city and again became the capital of united Germany.

The East German government started to falter in May 1989, when the removal of Hungary's border fence with Austria opened a hole in the Iron Curtain. The border was still closely guarded, but the Pan-European Picnic and the indecisive reaction of the rulers of the Eastern Bloc set in motion an irreversible peaceful movement. It allowed an exodus of thousands of East Germans fleeing to West Germany via Hungary. The Peaceful Revolution, a series of protests by East Germans, led to the GDR's first free elections on 18 March 1990 and to the negotiations between the GDR and FRG that culminated in a Unification Treaty. Other negotiations between the GDR and FRG and the four occupying powers produced the so-called "Two Plus Four Treaty" (Treaty on the Final Settlement with Respect to Germany), granting full sovereignty to a unified German state, whose two parts were previously bound by a number of limitations stemming from their post-World War II status as occupied regions.

The 1945 Potsdam Agreement had specified that a full peace treaty concluding World War II, including the exact delimitation of Germany's postwar boundaries, was required to be "accepted by the Government of Germany when a government adequate for the purpose is established." The Federal Republic had always maintained that no such government could be said to have been established until East and West Germany had been united within a free democratic state; but, in 1990, a range of opinions continued to be maintained over whether a unified West Germany, East Germany, and Berlin could be said to represent "Germany as a whole" for this purpose. The key question was whether a Germany that remained bounded to the east by the Oder–Neisse line (the international border with Poland) could act as a "united Germany" in signing the peace treaty without qualification. Under the "Two Plus Four Treaty", both the Federal Republic and the Democratic Republic committed themselves and their unified continuation to the principle that their joint pre-1990 boundaries constituted the entire territory that could be claimed by a Government of Germany, and hence that there were no further lands outside those boundaries that were parts of Germany as a whole.

The post-1990 united Germany is not a successor state, but an enlarged continuation of the former West Germany. The enlarged Federal Republic of Germany retained the West German seats in international organizations including the European Economic Community (later the European Union), NATO, and the United Nations. Memberships in the Warsaw Pact and other international organizations to which East Germany belonged ended because East Germany ceased to exist.