Sigismund II Augustus (1 August 1520 – 7 July 1572) was King of Poland and Grand Duke of Lithuania, the son of Sigismund I the Old, whom Sigismund II succeeded in 1548. He was the first ruler of the Polish–Lithuanian Commonwealth and the last male monarch from the Jagiellonian dynasty.

Sigismund was the only son of Italian-born Bona Sforza and Sigismund the Old. From the beginning he was groomed and extensively educated as a successor. In 1529 he was crowned vivente rege while his father was still alive. Sigismund Augustus continued a tolerance policy towards minorities and maintained peaceful relations with neighbouring countries, with the exception of the Northern Seven Years' War which aimed to secure Baltic trade. Under his patronage, culture flourished in Poland; he was a collector of tapestries from the Low Countries and collected military memorabilia as well as swords, armours and jewellery. Sigismund Augustus' rule is widely considered as the apex of the Polish Golden Age; he established the first regular Polish navy and the first regular postal service in Poland, known today as Poczta Polska. In 1569 he oversaw the signing of the Union of Lublin between Poland and the Grand Duchy of Lithuania, which formed the Polish–Lithuanian Commonwealth and introduced an elective monarchy.

Sigismund Augustus married three times; his first wife, Elizabeth of Austria, died in 1545 at just eighteen. He was then involved in several relationships with mistresses, the most famous being Barbara Radziwiłł, who became Sigismund's second wife and Queen of Poland in spite of his mother's disapproval. The marriage was deemed scandalous and was fiercely opposed by the royal court and the nobility. Barbara died five months after her coronation, presumably due to ill health, however, rumours circulated that she was poisoned. Sigismund finally wedded Catherine of Austria, but remained childless throughout his life.

Sigismund Augustus was the last male member of the Jagiellons. Following the death of his sister Anna in 1596 the Jagiellonian dynasty came to an end.