Stanisław August Poniatowski (also Stanisław II August; born Stanisław Antoni Poniatowski; January 17, 1732 – February 12, 1798) is the last King and Grand Duke of the Polish-Lithuanian Commonwealth (1764–95).

A Polish noble of the Ciołek lineage, he is the son of Count Stanisław Poniatowski, Castellan of Kraków, and Princess Konstancja Czartoryska; brother of Michał Jerzy Poniatowski (1736–94), Primate of Poland; and uncle to Prince Józef Poniatowski, (1763–1813).

While at the royal court in Saint Petersburg, Russia, he becomes romantically involved with the twenty-six-year-old future Empress Catherine Alexeievna (Catherine the Great), three years his senior.

With her support, in 1764 he is elected king of Poland.

Against expectations, he attempts to reform and strengthen the ailing Commonwealth.

His efforts meet with external opposition from Russia and Austria, both interested in keeping the Commonwealth weak; and from internal conservative interests, which sees reforms as threatening their traditional liberties and prerogatives.

The defining crisis of his early reign is the War of the Bar Confederation (1768–72), which leads to the First Partition of Poland (1772).

The latter part of his reign sees reforms wrought by the Great Sejm (1788–1792) and the Constitution of May 3, 1791.

These reforms are overthrown by the 1792 Targowica Confederation and War in Defense of the Constitution, leading directly to the Second Partition of Poland (1793), the Kościuszko Uprising (1794) and the final Third Partition of Poland (1795), marking the end of the Commonwealth.

Poniatowski abdicates soon afterward and spends the last years of his life in semi-captivity in Saint Petersburg.

He remains a controversial figure in Polish history.

Recognized as a great patron of the arts and sciences and a supporter of reforms, he is also remembered as the last king of the Commonwealth, who failed to prevent its destruction.