Sigismund III Vasa (Polish: Zygmunt III Waza, Lithuanian: Žygimantas Vaza; 20 June 1566 – 30 April 1632 N.S.) was King of Poland and Grand Duke of Lithuania from 1587 to 1632 and, as Sigismund, King of Sweden from 1592 to 1599. He was the first Polish sovereign from the House of Vasa. Religiously zealous, he imposed Catholicism across the vast realm, and his crusades against neighbouring states marked Poland's largest territorial expansion. As an enlightened despot, he presided over an era of prosperity and achievement, further distinguished by the transfer of the country's capital from Kraków to Warsaw.

Sigismund was the son of King John III of Sweden and his first wife, Catherine Jagiellon, daughter of King Sigismund I of Poland. Elected monarch of the Polish–Lithuanian Commonwealth in 1587, he sought to unify Poland and Sweden under one Catholic kingdom, and when he succeeded his deceased father in 1592 the Polish–Swedish union was created. Opposition in Protestant Sweden caused a war against Sigismund headed by Sigismund's uncle Charles IX, who deposed him in 1599.

Sigismund attempted to hold absolute power in all his dominions and frequently undermined parliament. He suppressed internal opposition, strengthened Catholic influence and granted privileges to the Jesuits, whom he employed as advisors and spies during the Counter-Reformation. He actively interfered in the affairs of neighbouring countries; his successful invasion of Russia during the Time of Troubles resulted in the seizure of Smolensk and occupation of Moscow, resulting in Poland's historical greatest territorial extent. Sigismund's army also defeated the Ottoman forces in southeastern Europe, which hastened the downfall of Sultan Osman II. However, the Polish–Swedish conflict had a less favourable outcome. After a series of skirmishes ending in a truce, King Gustavus Adolphus of Sweden launched a campaign against the Commonwealth and annexed parts of Polish Livonia.

Sigismund remains a controversial figure in Poland. He is one of the country's most recognisable monarchs. His long reign partially coincided with the Polish Golden Age, the apex in the prestige, power and economic influence of the Polish–Lithuanian Commonwealth. On the other hand, it was also during his rule that the seeds of decline surfaced. Considerable contributions to the arts and architecture as well as military victories were tarnished by intrigues and religious persecutions. He was commemorated in Warsaw by Sigismund's Column, one of the city's chief landmarks and the first secular monument in the form of a column in modern history. It was commissioned after Sigismund's death by his son and successor, Władysław IV.

Born on 20 June 1566 at Gripsholm Castle, Sigismund was the second child and only son of Catherine Jagiellon and Grand Duke John, who was a son of King Gustav I of Sweden. The couple was being held prisoner at Gripsholm since 1563 when John staged a failed rebellion against his deranged brother Eric XIV of Sweden. Although Protestant Christians were growing political wing in Poland at the time, Sigismund was raised as a Catholic. His mother Catherine was the daughter of Polish king Sigismund the Old and Bona Sforza of Milan, all of whom where practicing Catholics. Sigismund's older sister Isabella died aged two in 1566. His younger sister Anna was a Lutheran, but the close relationship between the two siblings remained unchanged until her death in 1625.

In October 1567, Sigismund and his parents were released from prison at the request of his uncle Charles. In January 1569, Eric XIV was deposed and Sigismund's father ascended the throne of Sweden as John III. He maintained good relations with his father despite John's second marriage to Gunilla Bielke, a Protestant noble lady of lower status and Catherine's former maid of honour. In 1589, Sigismund's half-brother John, the future Duke of Östergötland, was born.

As a child, Sigismund was tutored in both Polish and Swedish, thus making him bilingual. He was also proficient in German, Italian, and Latin. Catherine ensured that her son was educated in the spirit of Catholicism and Polish patriotism; the young prince was made aware of his blood connection to the Jagiellonian dynasty which ruled Poland in its finest period for two hundred years. Although Sigismund in his youth enjoyed reading and learning, observers did not acknowledge his intelligence. He was handsome, rather tall, and of slim build, but timid and an introvert who became heavily influenced by the teachings of the church. Nevertheless, Sigismund was undoubtedly multitalented and artistically inclined.

In 1587, Sigismund stood for election to the Polish throne after the death of Stephen Báthory. His candidacy was secured by Queen Dowager Anna and several elite magnates who considered him a native candidate as a descendant of the Jagiellons, though the election was openly questioned and opposed by the nobles politically associated with the Zborowski family. With the blessing of primate Stanisław Karnkowski and strong support from other people of influence he was duly elected ruler of the Polish–Lithuanian Commonwealth on 19 August 1587. His official name and title became "by the grace of God, king of Poland, grand duke of Lithuania, ruler of Ruthenia, Prussia, Masovia, Samogitia, Livonia and also hereditary king of the Swedes, Goths and Wends"; the latter titles being a reference to the fact that he was already the Crown Prince of Sweden, and thus would lawfully succeed to the throne of Sweden upon the death of his father.