The Weimar Republic ( , ˈvaɪ̯marɐ repuˈbliːk) is the nickname given to the German state from 1919 to 1933. It was dubbed the "Weimar Republic" by historians in honor of the city of Weimar, where a national assembly convened to write and adopt a new constitution (which became effective on August 11, 1919) for the German Reich, following the nation's defeat in World War I.

Despite its political form, the new republic was still officially known as the Deutsches Reich in German, rendered as "German Empire" or "German Reich" in English. The name Weimar Republic is an invention of historians, and was not used during its existence; the common short form remained "Germany". The Weimar Republic was established in February 1919 in defeated Germany and lasted until March 1933, when the state's interior was replaced with Hitler's so-called "Third Reich" (see Nazi Germany).

This first attempt to establish a liberal democracy in Germany happened during a time of civil conflict, and failed with the ascent of Adolf Hitler and the Nazi Party. Although technically the 1919 constitution was not invalidated until after World War II, the legal measures taken by the Nazi government in 1933 (commonly known as Gleichschaltung) destroyed the mechanisms of a typical democratic system, so that 1933 is usually cited as the end of the Weimar Republic.