John III (Swedish: Johan III, Finnish: Juhana III; 20 December 1537 – 17 November 1592) 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 ruler of Finland, as Duke John from 1556 to 1563. In 1581 he assumed also 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.
His first wife was Catherine Jagellonica of the Polish-Lithuanian ruling family, and their son Sigismund eventually ascended both the Polish-Lithuanian and Swedish thrones.