Sigismund was born on 20 June 1566 to Catherine Jagiellon and the Grand Duke John of Finland at Gripsholm.
His parents, at the time, were being held prisoner by King Eric XIV, but despite the Protestant domination of Sweden young Sigismund was raised as a Roman Catholic.
His mother Catherine was the daughter of Polish king Sigismund I the Old and Queen Bona Sforza of Italy.
In 1567 Sigismund and his parents were released from prison.
A year later, in 1568, Erik XIV was deposed and Sigismund's father ascended to the throne of Sweden as King John III.
From 1568 onward Sigismund was the Crown Prince of Sweden.

In 1587 Sigismund stood for election to the Polish throne after the death of king Stephen Bathory.
He was supported by his aunt Queen Anna, Hetman Jan Zamoyski and those nobles who considered him a Piast (native candidate), but he was opposed by the nobles loyal to the Zborowski family.
With such a strong support from the elite families and people of influence he was duly elected ruler of the Polish–Lithuanian Commonwealth on 19 August 1587 with the blessings of the primate of Poland Stanisław Karnkowski.
From that time 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 later 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.
Sigismund was born on 20 June 1566 to Catherine Jagiellon and the Grand Duke John of Finland at Gripsholm. His parents, at the time, were being held prisoner by King Eric XIV, but despite the Protestant domination of Sweden young Sigismund was raised as a Roman Catholic. His mother Catherine was the daughter of Polish king Sigismund I the Old and Queen Bona Sforza of Italy. In 1567 Sigismund and his parents were released from prison. A year later, in 1568, Erik XIV was deposed and Sigismund's father ascended to the throne of Sweden as King John III. From 1568 onward Sigismund was the Crown Prince of Sweden.
In 1587 Sigismund stood for election to the Polish throne after the death of king Stephen Bathory. He was supported by his aunt Queen Anna, Hetman Jan Zamoyski and those nobles who considered him a Piast (native candidate), but he was opposed by the nobles loyal to the Zborowski family. With such a strong support from the elite families and people of influence he was duly elected ruler of the Polish–Lithuanian Commonwealth on 19 August 1587 with the blessings of the primate of Poland Stanisław Karnkowski. From that time 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 later 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.
Sigismund III Vasa