Wilhelm II (Friedrich Wilhelm Viktor Albert; 27 January 1859 4 June 1941) was the last German Emperor (German: Kaiser) and King of Prussia, reigning from 15 June 1888 until his abdication on 9 November 1918. Despite strengthening the German Empire's position as a great power by building a powerful navy, his tactless public statements and erratic foreign policy greatly antagonized the international community and are considered by many to be one of the underlying causes of World War I. When the German war effort collapsed after a series of crushing defeats on the Western Front in 1918, he was forced to abdicate, thereby marking the end of the German Empire and the House of Hohenzollern's 300-year reign in Prussia and 500-year reign in Brandenburg.

Wilhelm II was the son of Prince Frederick William of Prussia and Victoria, German Empress Consort. His father was the son of Wilhelm I, German Emperor, and his mother was the eldest daughter of Queen Victoria of the United Kingdom and Prince Albert of Saxe-Coburg and Gotha. Wilhelm's grandfather, Wilhelm I, died in March 1888. His father became Emperor Frederick III, but died just 99 days later; in what is called the Year of the Three Emperors, Wilhelm II ascended the throne of the German Empire in June 1888.

In March 1890, Wilhelm II dismissed the German Empire's powerful longtime Chancellor, Otto von Bismarck, and assumed direct control over his nation's policies, embarking on a bellicose "New Course" to cement its status as a leading world power. Over the course of his reign, the German colonial empire acquired new territories in China and the Pacific (such as Kiautschou Bay, the Northern Mariana Islands, and the Caroline Islands) and became Europe's largest manufacturer. However, Wilhelm often undermined such progress by threatening and making tactless statements towards other countries without first consulting his ministers. Likewise, his regime did much to alienate itself from other great powers by initiating a massive naval build-up, contesting French control of Morocco, and building a railway through Baghdad that challenged Britain's dominion in the Persian Gulf. By the second decade of the 20th century, Germany could rely only on significantly weaker nations such as Austria-Hungary and the declining Ottoman Empire as allies.

Wilhelm's reign culminated in Germany's guarantee of military support to Austria-Hungary during the crisis of July 1914, one of the immediate causes of World War I. A lax wartime leader, Wilhelm left virtually all decision-making regarding strategy and organisation of the war effort to the German Army's Great General Staff. By August 1916, this broad delegation of power gave rise to a de facto military dictatorship that dominated national policy for the rest of the conflict. Despite emerging victorious over Russia and obtaining significant territorial gains in Eastern Europe, Germany was forced to relinquish all its conquests after a decisive defeat on the Western Front in the fall of 1918. Losing the support of his country's military and many of his subjects, Wilhelm was forced to abdicate during the German Revolution of 19181919. The revolution converted Germany from a monarchy into an unstable democratic state known as the Weimar Republic. Wilhelm fled to exile in the Netherlands where he remained during its occupation by Nazi Germany in 1940. He died there in 1941.

Kaiser is the German word for "emperor" (female Kaiserin). In general, the German title in principal applies to rulers anywhere in the world above the rank of king (König). In English, the (untranslated) word Kaiser is mainly applied to the emperors of the unified German Empire (1871–1918) and the emperors of the Austrian Empire (1804-1918). During the First World War, anti-German sentiment was at its zenith; the term Kaiser—especially as applied to Wilhelm II, German Emperor—thus gained considerable negative connotations in English-speaking countries.

Especially in Central Europe, between northern Italy and southern Poland, between western Austria and western Ukraine and in Bavaria, Emperor Franz Joseph I is still associated with "Der Kaiser (the emperor)" today. As a result of his long reign from 1848 to 1916 and the associated Golden Age before the First World War, this title often has still a very high historical respect in this geographical area.