Stanisław I Leszczyński (Polish pronunciation: [staˈniswaf lɛʂˈtʂɨɲski]) (Lithuanian: Stanislovas Leščinskis; French: Stanislas Leszczynski) (20 October 1677 – 23 February 1766) was King of Poland, Duke of Lorraine and a count of the Holy Roman Empire (a rank bestowed by Emperor Frederick III on the Leszczyński family).

Born in Lwów in 1677, he was the son of Rafał Leszczyński, voivode of Poznań Voivodeship, and Anna Katarzyna Jabłonowska. He married Katarzyna Opalińska, by whom he had a daughter, Maria, who became Queen-Consort of France as wife of Louis XV. In 1697, as Cup-bearer of Poland, he signed the confirmation of the articles of election of August II the Strong. In 1703 he joined the Lithuanian Confederation, which the Sapiehas with the aid of Sweden had formed against August.

The following year, Stanisław was selected by Charles XII of Sweden after a successful Swedish invasion of Poland, to supersede Augustus II, who was hostile towards the Swedes. Leszczyński was a young man of blameless antecedents, respectable talents, and came from an ancient family, but certainly without sufficient force of character or political influence to sustain himself on so unstable a throne.

Nevertheless, with the assistance of a bribing fund and an army corps, the Swedes succeeded in procuring his election by a scratch assembly of half a dozen castellans and a few score of gentlemen on 12 July 1704. A few months later, Stanisław was forced by a sudden inroad of August to seek refuge in the Swedish camp, but finally on 24 September 1705, he was crowned king with great splendor. Charles himself supplied his nominee with a new crown and scepter in lieu of the ancient Polish regalia, which had been carried off to Saxony by August. During this time the King of Sweden sent Peter Estenberg to King Stanislaw to act as an ambassador and correspondence secretary. The Polish king's first act was to cement an alliance with Charles XII whereby the Polish-Lithuanian Commonwealth engaged to assist Sweden against the Russian tsar. Stanisław did what he could to assist his patron. Thus, he induced Ivan Mazepa, the Cossack hetman, to desert Peter the Great at the most critical period of the Great Northern War between Russia and Sweden, and Stanisław placed a small army corps at the disposal of the Swedes. But Stanisław depended so entirely on the success of Charles' arms that after the Battle of Poltava (1709) Stanisław's authority vanished as a dream at the first touch of reality. During this period Stanisław resided in the town of Rydzyna.

The vast majority of Poles hastened to repudiate Stanisław and make their peace with August. Henceforth a mere pensioner of Charles XII, Stanisław accompanied Krassow's army corps in its retreat to Swedish Pomerania. On the restoration of Augustus, Stanisław resigned the Polish Crown (though he retained the royal title) in exchange for the little principality of Zweibrücken. In 1716, an assassination was attempted by a Saxon officer, Lacroix, but Stanisław was saved by Stanisław Poniatowski, father of the future king. Stanisław Leszczyński then resided at Wissembourg in Alsace. In 1725, he had the satisfaction of seeing his daughter Maria become the consort of Louis XV and queen of France. From 1725 to 1733, Stanisław lived at the Château de Chambord.

Stanislaw's son-in-law Louis XV supported his claims to the Polish throne after the death of August II the Strong in 1733, which led to the War of the Polish Succession. In September 1733, Stanisław himself arrived at Warsaw, having traveled night and day through central Europe disguised as a coachman. On the following day, despite many protests, Stanisław was duly elected King of Poland for the second time. However, Russia was opposed to any nominee of France and Sweden. Russia protested against his election at once, in favor of the new Elector of Saxony, as being the candidate of her Austrian ally.

On 30 June 1734, a Russian army of 20,000 under Peter Lacy, after proclaiming August III the Saxon at Warsaw, proceeded to besiege Stanisław at Danzig, where he was entrenched with his partisans (including the Primate and the French and Swedish ministers) to await the relief that had been promised by France.

The siege began in October 1734. On 17 March 1735, Marshal Münnich superseded Peter Lacy, and on 20 May 1735 the long-expected French fleet appeared and disembarked 2,400 men on Westerplatte. A week later, this little army gallantly attempted to force the Russian entrenchments, but was finally compelled to surrender. This was the first time that France and Russia had met as foes in the field. On 30 June 1735, Danzig capitulated unconditionally, after sustaining a siege of 135 days which cost the Russians 8,000 men.

Disguised as a peasant, Stanisław had contrived to escape two days before. He reappeared at Königsberg, whence he issued a manifesto to his partisans which resulted in the formation of a confederation on his behalf, and the despatch of a Polish envoy to Paris to urge France to invade Saxony with at least 40,000 men. In Ukraine too, Count Nicholas Potocki kept on foot to support Stanisław a motley host of 50,000 men, which was ultimately scattered by the Russians.

On 26 January 1736, Stanisław again abdicated the throne, but received in compensation the Duchy of Lorraine and of Bar, which was to revert to France on his death. In 1738, he sold his estates of Rydzyna and Leszno to Count (later Prince) Alexander Joseph Sułkowski. He settled at Lunéville, founded there in 1750 both the Académie de Stanislas and Bibliothèque municipale de Nancy, and devoted himself for the rest of his life to science and philanthropy, engaging most notably in controversy with Rousseau.

Stanisław was still living at the time of the birth of his great-great-granddaughter, Archduchess Maria Theresa of Austria, in 1762. He died in 1766, aged 88. His works include Œuvres du philosophe bienfaisant, Paris, 1763, 1866.