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 and Grand Duke of Finland from 1592 to 1599. He was the first Polish sovereign from the House of Vasa. A religious zealot, he imposed Roman Catholic doctrine 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 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 invasion of Russia during the Time of Troubles resulted in brief control over Moscow and seizure of Smolensk. 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. One of the country's most recognisable monarchs, his long reign 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.