Sigismund III Vasa, who was already King of Poland, succeeded his father John III as King of Sweden.

Sigismund III Vasa 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.

Poland was ruled at various times either by dukes and princes or by kings. During the latter period, a tradition of free election of monarchs made it a uniquely electable position in Europe.

John III was King of Sweden from 1569 until his death. He attained the Swedish throne after a rebellion against his half-brother Erik XIV. He is mainly remembered for his attempts to close the gap between the newly established Lutheran Church of Sweden and the Catholic Church, as well as his conflict with and possible murder of his brother.

This list records the monarchs of Sweden, from the late Viking Age to the present day. Sweden has continuously been a monarchy since the country's consolidation in the Viking Age and early Middle Ages, for over a thousand years. The incumbent royal dynasty of Sweden is the House of Bernadotte, established on the throne in 1818.