Augustus III (German: August III.; Polish: August III Sas – "the Saxon"; Lithuanian: Augustas III; 17 October 1696 – 5 October 1763) was King of Poland and Grand Duke of Lithuania from 1733 until 1763, as well as Elector of Saxony in the Holy Roman Empire where he was known as Frederick Augustus II (German: Friedrich August II.).

He was the only legitimate son of Augustus II the Strong, and converted to Catholicism in 1712 to secure his candidacy for the Polish throne. In 1719 he married Maria Josepha, daughter of Joseph I, Holy Roman Emperor, and became elector of Saxony following his father's death in 1733. Augustus was able to gain the support of Charles VI by agreeing to the Pragmatic Sanction of 1713 and also gained recognition from Russian Empress Anna by supporting Russia's claim to the region of Courland. He was elected king of Poland by a small minority on 5 October 1733 and subsequently banished the former Polish king Stanisław I. He was crowned in Kraków on 17 January 1734.

Augustus was supportive of Austria against Prussia in the War of Austrian Succession and again in the Seven Years' War (1756), both of which resulted in Saxony being defeated and occupied by Prussia. In Poland, his rule was marked by the increasing influence of the Czartoryski and Poniatowski families, and by the intervention of Catherine the Great in Polish affairs. His rule deepened the social anarchy in Poland and increased the country's dependence on its neighbours, notably Prussia, Austria, and Russia. The Russian Empire prevented him from installing his family on the Polish throne, supporting instead the aristocrat Stanisław August Poniatowski, the lover of Catherine the Great. Throughout his reign, Augustus was known to be more interested in ease and pleasure than in the affairs of state; this notable patron of the arts left the administration of Saxony and Poland to his chief adviser, Heinrich von Brühl, who in turn left Polish administration chiefly to the powerful Czartoryski family.

Royal titles in Latin: Augustus tertius, Dei gratia rex Poloniae, magnus dux Lithuaniæ, Russiæ, Prussiæ, Masoviæ, Samogitiæ, Kijoviæ, Volhiniæ, Podoliæ, Podlachiæ, Livoniæ, Smolensciæ, Severiæ, Czerniechoviæque, nec non-hæreditarius dux Saxoniæ et princeps elector etc.

English translation: August III, by the grace of God, King of Poland, Grand Duke of Lithuania, Ruthenia, Prussia, Masovia, Samogitia, Kiev, Volhynia, Podolia, Podlachia, Livonia, Smolensk, Severia, Chernihiv, and also hereditary Duke of Saxony and Prince-Elector, etc.

Augustus was born 17 October 1696 in Dresden, the only legitimate son of Augustus II the Strong, Prince-Elector of Saxony and ruler of the Polish–Lithuanian Commonwealth who belonged to the Albertine branch of the House of Wettin. His mother was Christiane Eberhardine of Brandenburg-Bayreuth, daughter of Christian Ernst, Margrave of Brandenburg-Bayreuth. Unlike his father, Christiane remained a fervent Protestant throughout her life and never set foot in Catholic Poland during her 30-year service as queen consort. Despite the pressure from Augustus II, she was never crowned at Wawel in Kraków and purely held a titular title of queen. This move was viewed by the Polish nobility as a provocation and from the beginning the prince was treated with prejudice in Poland.

From his early years, Augustus was groomed to succeed as king of Poland-Lithuania; best tutors were hired from across the continent and the prince studied Polish, German, French and Latin. He was taught Russian, but was unable to speak it fluently, as well as exact sciences including mathematics, chemistry and geography. He also practiced equestrianism in his youth. While his father spent time in Poland, the young Augustus was left in the care of his grandmother, Princess Anna Sophie of Denmark, who initially raised him Lutheran. This was particularly unfavourable for the Poles, who wouldn't accept or tolerate a Protestant monarch. As a consequence, a troubled Augustus II organized a tour of Catholic countries in Europe for his son which he hoped would bring him closer to Catholicism and break the bond between him and his controlling grandmother. In Venice, the Polish entourage thwarted a kidnapping attempt organized by British agents of Queen Anne in order to prevent him from converting. He also witnessed the coronation of Charles VI in 1711 after the death of his brother and predecessor, Joseph I.

Augustus eventually converted to Catholicism in November 1712 while extensively touring Italy, and its cultural and religious heritage. He was then under the supervision of the Jesuits, who certainly contributed to the cause. The public announcement of conversion in 1717 triggered discontent among the Protestant Saxon aristocracy. Faced with a hereditary Catholic succession for Saxony, Prussia and Hanover attempted to oust Saxony from the directorship of the Protestant body in the Reichstag of the Holy Roman Empire, but Saxony managed to retain the directorship.