Stanisław I Leszczyński (Polish pronunciation: [staˈɲiswaf lɛʂˈtʂɨj̃skʲi]; Lithuanian: Stanislovas Leščinskis; French: Stanislas Leszczynski; 20 October 1677 – 23 February 1766), also Anglicized and Latinized as Stanislaus I, was twice King of Poland and Grand Duke of Lithuania, and at various times Prince of Deux-Ponts, Duke of Bar and Duke of Lorraine.
During the Great Northern War, multiple candidates had emerged at the death John III Sobieski for the elective kingship of Poland (which also included the Grand Duchy of Lithuania as part of the Poland-Lithuanian Commonwealth). Backed by powerful neighbors in Russia and Austria, the Sejm elected August the Strong, Elector of Saxony to succeed John III in 1697 as August II. Russia's primary antagonist in the Great Northern War, Sweden had supported Stanisław Leszczyński for the throne, and after defeating a combined army of Saxon and Polish-Lithuanian forces, deposed August II and installed Leszczyński as Stanisław I in 1704.
In 1709, Charles XII of Sweden, Stanisław's main supporter, suffered a defeat by the Russians at the Battle of Poltava, and was subsequently driven into exile in the Ottoman Empire. As a result, Augustus II returned to the throne, and while Charles served his exile in the Ottoman Empire, Stanisław accepted the rule of the tiny state of Deux Ponts (Zweibrücken), a small state of the Holy Roman Empire which was in personal union with Sweden and located near the region of Alsace. After Charles's death in 1719, he moved to nearby Wissembourg in Alsace. In 1725, his daughter Marie Leszczyńska married Louis XV of France.
The death of August II sparked the War of the Polish Succession in 1733. As had happened on the death of John III, foreign intrigue and influence plagued the Sejm election. Despite the presence of Russian troops in the country, the Sejm, with support of the French, elected Stanisław to succeed August II, while the Russians encouraged a group of break-away nobles to hold their own election, selecting instead Frederick August of Saxony, son of August II, to the kingship. War broke out almost immediately, evolving into a proxy war between the Bourbon and Habsburg dynasties and their supporters, with the Bourbon faction led by France and Spain, with their allies Savoy and Sweden, while the Habsburg faction was led by Austria and their allies Russia, Prussia, and Saxony. After two years of fighting across the entire continent, a cease-fire was declared in 1735. Stanislaus officially abdicated in January 1736, and the Peace of Vienna was promulgated in 1738, whereby August was officially recognized as King of Poland, and Stanisław was compensated for losing the throne a second time with the duchies of Bar and Lorraine, both of which were nominally part of the Holy Roman Empire at the time.
While Duke of Lorraine, Stanislas lived out his remaining years at a country estate in Lunéville, and actively ruled Lorraine and Barrois, sponsoring numerous public works projects. Nearby, Nancy, the historic capital of Lorraine, has a Place Stanislas (Stanisław Square) named in his honour, much of which was developed during his reign. He also took up political philosophy, engaging in discourse with other Enlightenment figures such as Jean Jacques Rousseau, and wrote philosophical treatises in his native Polish, making him a figure within the Polish Enlightenment. When he died in 1766, his titles passed to his son-in-law, Louis XV.
His retaking of the Polish throne in 1733 formed the backdrop for Un giorno di regno, an opera by composer Giuseppe Verdi and librettist Felice Romani, which premiered in 1840.