The Treaty of Versailles (French: Trait de Versailles; German: Versailler Vertrag, pronounced [vza ftak] (listen)) was the most important of the peace treaties of World War I. It ended the state of war between Germany and the Allied Powers. It was signed on 28 June 1919 in the Palace of Versailles, exactly five years after the assassination of Archduke Franz Ferdinand, which led to the war. The other Central Powers on the German side signed separate treaties.

Although the armistice of 11 November 1918 ended the actual fighting, it took six months of Allied negotiations at the Paris Peace Conference to conclude the peace treaty. The treaty was registered by the Secretariat of the League of Nations on 21 October 1919.

Of the many provisions in the treaty, one of the most important and controversial was: "The Allied and Associated Governments affirm and Germany accepts the responsibility of Germany and her allies for causing all the loss and damage to which the Allied and Associated Governments and their nationals have been subjected as a consequence of the war imposed upon them by the aggression of Germany and her allies." The other members of the Central Powers signed treaties containing similar articles.

This article, Article 231, became known as the War Guilt clause. The treaty required Germany to disarm, make ample territorial concessions, and pay reparations to certain countries that had formed the Entente powers. In 1921 the total cost of these reparations was assessed at 132 billion gold marks (then $31.4 billion or £6.6 billion, roughly equivalent to US$442 billion or UK£284 billion in 2022).

Prominent economists such as John Maynard Keynes declared the treaty too harsha "Carthaginian peace"and said the reparations were excessive and counter-productive. On the other hand, prominent Allied figures such as French Marshal Ferdinand Foch, criticized the treaty for treating Germany too leniently. This is still the subject of ongoing debate by historians and economists.

The result of these competing and sometimes conflicting goals among the victors was a compromise that left no one satisfied. In particular, Germany was neither pacified nor conciliated, nor was it permanently weakened. The problems that arose from the treaty would lead to the Locarno Treaties, which improved relations between Germany and the other European powers, and the re-negotiation of the reparation system resulting in the Dawes Plan, the Young Plan, and the indefinite postponement of reparations at the Lausanne Conference of 1932. The treaty has sometimes been cited as a cause of World War II: although its actual impact was not as severe as feared, its terms led to great resentment in Germany which powered the rise of the Nazi Party.

Although it is often referred to as the "Versailles Conference", only the actual signing of the treaty took place at the historic palace. Most of the negotiations were in Paris, with the "Big Four" meetings taking place generally at the French Ministry of Foreign Affairs on the Quai d'Orsay.

The Upper Silesia plebiscite was a plebiscite mandated by the Versailles Treaty and carried out on 20 March 1921 to determine ownership of the province of Upper Silesia between Weimar Germany and Poland. The region was ethnically mixed with both Germans and Poles; according to prewar statistics, ethnic Poles formed 60 percent of the population. Under the previous rule by the German Empire, Poles claimed they had faced discrimination, making them effectively second class citizens. The period of the plebiscite campaign and inter-Allied occupation was marked by violence. There were three Polish uprisings, and German volunteer paramilitary units came to the region as well.

The area was policed by French, British, and Italian troops, and overseen by an Inter-Allied Commission. The Allies planned a partition of the region, but a Polish insurgency took control of over half the area. The Germans responded with volunteer paramilitary units from all over Germany, which fought the Polish units. In the end, after renewed Allied military intervention, the final position of the opposing forces became, roughly, the new border. The decision was handed over to the League of Nations, which confirmed this border, and Poland received roughly one third of the plebiscite zone by area, including the greater part of the industrial region.After the referendum, on 20 October 1921, a conference of ambassadors in Paris decided to divide the region. Consequently, the German-Polish Accord on East Silesia (Geneva Convention), a minority treaty, was concluded on 15 May 1922 which dealt with the constitutional and legal future of Upper Silesia that had partly become Polish territory.