The Weimar Republic was a historical period of the German state from 9 November 1918 to 23 March 1933, during which it was a constitutional republic for the first time in history. The state was officially named the German Reich; it is also referred to, and unofficially proclaimed itself, as the German Republic. The period's informal name is derived from the city of Weimar, where the republic's constituent assembly took place. In English, the republic was usually simply called "Germany", with "Weimar Republic" (a term introduced by Adolf Hitler in 1929) not commonly used until the 1930s. The Weimar Republic had a semi-presidential system.

Toward the end of the First World War (1914–1918), Germany was exhausted and sued for peace in desperate circumstances. Awareness of imminent defeat sparked a revolution, the abdication of Kaiser Wilhelm II, the proclamation of the Weimar Republic on 9 November 1918, and formal cessation of hostilities with the Allies by the Armistice of 11 November 1918.

In its initial years, grave problems beset the Republic, such as hyperinflation and political extremism, including political murders and two attempted coups d'état by contending paramilitaries; internationally, it suffered isolation, reduced diplomatic standing and contentious relationships with the great powers. By 1924, a great deal of monetary and political stability was restored, and the republic enjoyed relative prosperity for the next five years; this period, sometimes known as the Golden Twenties, was characterized by significant cultural flourishing, social progress, and gradual improvement in foreign relations. Under the Locarno Treaties of 1925, Germany moved toward normalizing relations with its neighbors, recognizing most territorial changes under the 1919 Treaty of Versailles and committing never to go to war. The following year, it joined the League of Nations, which marked its reintegration into the international community. Nevertheless, especially on the political right, there remained strong and widespread resentment against the treaty and those who had signed and supported it.

The Great Depression of October 1929 severely affected Germany's tenuous progress; high unemployment and subsequent social and political unrest led to the collapse of Chancellor Hermann Müller's grand coalition and the beginning of the presidential cabinets. From March 1930 onwards, President Paul von Hindenburg used emergency powers to back chancellors Heinrich Brüning, Franz von Papen and Kurt von Schleicher. The Great Depression, exacerbated by Brüning's policy of deflation, led to a surge in unemployment. On 30 January 1933, Hindenburg appointed Adolf Hitler as chancellor to head a coalition government; his Nazi Party held two out of ten cabinet seats. Von Papen, as vice-chancellor and Hindenburg's confidant, was to serve as the éminence grise who would keep Hitler under control; these intentions severely underestimated Hitler's political ambitions. By the end of March 1933, the Reichstag Fire Decree and the Enabling Act of 1933 were used in the perceived state of emergency to effectively grant the new chancellor broad power to act outside parliamentary control. Hitler promptly used these powers to thwart constitutional governance and suspend civil liberties, which brought about the swift collapse of democracy at the federal and state level, and the creation of a one-party dictatorship under his leadership.

Until the end of World War II in Europe in 1945, the Nazis governed Germany under the pretense that all the extraordinary measures and laws they implemented were constitutional; notably, there was never an attempt to replace or substantially amend the Weimar Constitution. Nevertheless, Hitler's seizure of power (Machtergreifung) had effectively ended the republic, replacing its constitutional framework with Führerprinzip, the principle that "the Führer's word is above all written law".

The Weimar Republic is so called because the Weimar National Assembly that adopted its constitution met in Weimar from 6 February to 11 August 1919.

Even though the National Assembly chose to retain the old name Deutsches Reich (Art. 1 of the Constitution), hardly anyone used it during the Weimar period, and no single name for the new state gained widespread acceptance. To the right of the spectrum, the politically engaged rejected the new democratic model and were appalled to see the honor of the traditional word Reich associated with it. The Catholic Centre Party favored the term Deutscher Volksstaat (German People's State), while on the moderate left, Chancellor Friedrich Ebert's Social Democratic Party of Germany preferred Deutsche Republik (German Republic). By the mid-1920s, most Germans referred to their government informally as the Deutsche Republik, but for many, especially on the right, the word "Republik" was a painful reminder of a government structure that they believed had been imposed by foreign statesmen and of the expulsion of Emperor Wilhelm II in the wake of a massive national humiliation.

The first recorded mention of the term Republik von Weimar (Republic of Weimar) came during a speech delivered by Adolf Hitler at a Nazi Party rally in Munich on 24 February 1929. A few weeks later, the term Weimarer Republik was first used, again by Hitler in a newspaper article. Only during the 1930s did the term become mainstream, both within and outside Germany.