Stanisław II Augustus (also Stanisław August Poniatowski; born Stanisław Antoni Poniatowski; 17 January 1732 – 12 February 1798), who reigned as King of Poland and Grand Duke of Lithuania from 1764 to 1795, was the last monarch of the Polish–Lithuanian Commonwealth. He remains a controversial figure in Polish history. Recognized as a great patron of the arts and sciences and an initiator and firm supporter of progressive reforms, he is also remembered as the King of the Commonwealth whose election was marred by Russian intervention. He is criticized primarily for his failure to stand against the partitions, and thus to prevent the destruction of the Polish state.

Having arrived at the Russian imperial court in Saint Petersburg in 1755, he became romantically involved with the future empress Catherine the Great (1762–1796). With her connivance, in 1764 he was elected King of Poland. Contrary to expectations, he attempted to reform and strengthen the ailing Commonwealth. His efforts met with external opposition from Prussia, Russia and Austria, all committed to keeping the Commonwealth weak. From within he was opposed by conservative interests, which saw reforms as threatening their traditional liberties and privileges.

The defining crisis of his early reign was the War of the Bar Confederation (1768–1772) that led to the First Partition of Poland (1772). The later part of his reign saw reforms wrought by the Great Sejm (1788–1792) and the Constitution of 3 May 1791. These reforms were overthrown by the 1792 Targowica Confederation and by the War in defence of the consitution, leading directly to the Second Partition of Poland (1793), the Kościuszko Uprising (1794) and the final and Third Partition of Poland (1795), marking the end of the Commonwealth. Stripped of all meaningful power, Poniatowski abdicated in November 1795 and spent the last years of his life a virtual captive in Saint Petersburg.

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