John III (, ) (20 December 1537 – 17 November 1592) was King of Sweden from 1568 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.