John III was King of Sweden from 1569 until his death. He was the son of King Gustav I of Sweden and his second wife Margaret Leijonhufvud. He was also, quite autonomously, the Duke of Finland from 1556 to 1563. In 1581 he assumed the title Grand Prince of Finland. He attained the Swedish throne after a rebellion against his half-brother Eric 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 murder of his brother.

The monarchy of Sweden is centred on the monarchical head of state of Sweden, by law a constitutional and hereditary monarchy with a parliamentary system. There have been kings in what now is the Kingdom of Sweden for more than a millennium. Originally an elective monarchy, it became a hereditary monarchy in the 16th century during the reign of Gustav Vasa, though virtually all monarchs before that belonged to a limited and small number of political families which are considered to be the royal dynasties of Sweden.