Józef Klemens Piłsudski (Polish: [ˈjuzɛf ˈklɛmɛns piwˈsutskʲi] (listen); 5 December 1867 – 12 May 1935) was a Polish statesman who served as the Chief of State (1918–1922) and First Marshal of Poland (from 1920). He was considered the de facto leader (1926–35) of the Second Polish Republic as the Minister of Military Affairs. After World War I, he held increasing dominance in Polish politics and was an active player in international diplomacy. He is viewed as a father of the Second Polish Republic re-established in 1918, 123 years after the final Partition of Poland in 1795.Seeing himself as a descendant of the culture and traditions of the Polish–Lithuanian Commonwealth, Piłsudski believed in a multi-ethnic Poland—"a home of nations" including indigenous ethnic and religious minorities. Early in his political career, Piłsudski became a leader of the Polish Socialist Party. Believing Poland's independence would be won militarily, he formed the Polish Legions. In 1914, he predicted a new major war would defeat the Russian Empire and the Central Powers. After World War I began in 1914, Piłsudski's Legions fought alongside Austria-Hungary against Russia. In 1917, with Russia faring poorly in the war, he withdrew his support for the Central Powers, and was imprisoned in Magdeburg by the Germans.

Piłsudski was Poland's Chief of State from November 1918, when Poland regained its independence, until 1922. From 1919–21 he commanded Polish forces in six wars that re-defined the country's borders. On the verge of defeat in the Polish–Soviet War in August 1920, his forces repelled the invading Soviet Russians at the Battle of Warsaw. In 1923, with a government dominated by his opponents, in particular the National Democrats, Piłsudski retired from active politics. Three years later he returned to power in the May Coup and became the strongman of the Sanacja regime. He focused on military and foreign affairs until his death in 1935, developing a cult of personality that has survived into the 21st century.

Some aspects of Piłsudski's administration, such as imprisoning his political opponents at Bereza Kartuska, remain controversial. Yet, he is highly esteemed in Polish memory and is regarded as a founder of modern independent Poland.