The dissolution of the Soviet Union (1988–1991) was the process of internal political, economic, and ethno-separatist disintegration within the USSR, which resulted in the end of its existence as a sovereign state. Being its ultimate unintended aftermath, it brought an end to General Secretary (later also President) Mikhail Gorbachev's effort to reform the Soviet political and economic system in an attempt to stop a period of political stalemate and economic backslide, known as the "Era of Stagnation" or simply as "Zastoy" (Russian for "stagnation" or "stasis"). In late 1991, amidst the catastrophic political crisis and several republics already breaking off the Union, the leaders of three of its founding members (the fourth one being dissolved in the 1930s) declared that the Soviet Union no longer existed, eight more republics joined their declaration shortly thereafter. Gorbachev had to resign and what was left of the parliament to formally acknowledge the collapse as a fait accompli.

The process began with growing unrest in the Union's various constituent national republics developing into an incessant political and legislative conflict between them and the central government. Estonia was the first Soviet republic to declare state sovereignty inside the Union on November 16, 1988. Lithuania was the first republic to declare independence from the Soviet Union by the Act of March 11, 1990 (not counting the autonomy of Nakhchivan, which had declared independence from both the Soviet Union and the Azerbaijan SSR a few weeks earlier, later rejoining Azerbaijan), with its Baltic neighbours and the Southern Caucasus republic of Georgia joining it in a course of two months.

The failure of the 1991 August Coup, when communist hardliners and military elites tried to overthrow Gorbachev and stop the failing reforms, led to the government in Moscow losing most of its influence, and many republics proclaiming independence in the following days and months. The secession of the Baltic states was recognized in September 1991. The Belovezh Accords were arbitrarily signed on December 8 by President Boris Yeltsin of Russia, President Kravchuk of Ukraine, and Chairman Shushkevich of Belarus, recognising each other's independence and creating the Commonwealth of Independent States (CIS) instead of the Soviet Union. Kazakhstan was the last nation to leave the Union, proclaiming independence on December 16. All the ex-Soviet republics, with the exception of Georgia and the Baltics, joined the CIS on December 21, signing the Alma-Ata Protocol.On December 25, Gorbachev resigned and turned over his presidential powers—including control of the nuclear launch codes—to Yeltsin, who was now the first president of the Russian Federation. That evening at 7:32 p.m., the Soviet red banner was lowered from the Kremlin for the last time and replaced with the Russian tricolour flag. The following day, the Declaration 142-Н of the Supreme Soviet's only functioning upper chamber, the Soviet of the Republics, recognised self-governing independence for the former Soviet republics, formally dissolving the Union. Both the Revolutions of 1989 in the Eastern Bloc and the dissolution of the Soviet Union marked the end of the Cold War.

In the aftermath of the Cold War, several of the former Soviet republics have retained close links with Russia and formed multilateral organizations such as CSTO, the CIS, the Eurasian Economic Community, the Union State, the Eurasian Customs Union, and the Eurasian Economic Union, for economic and military cooperation. On the other hand, the Baltic states and most of the former Warsaw Pact states became part of the European Union and joined NATO, while some of the other former Soviet republics like Ukraine, Georgia and Moldova have been publicly expressing interest in following the same path since the 1990s.