Sigismund II Augustus (Polish: Zygmunt II August, Lithuanian: Žygimantas II Augustas; 1 August 1520 – 7 July 1572) was the King of Poland and Grand Duke of Lithuania, the only son of Sigismund I the Old, whom Sigismund II succeeded in 1548. Married three times, the last of the Jagiellons remained childless, and through the Union of Lublin introduced a free elective monarchy. From the outset of his reign, Sigismund came into collision with the country's nobility, who had already begun curtailing the power of the great families. So violent was the agitation at Sigismund's first sejm (31 October 1548) that the deputies threatened to renounce their allegiance unless the King repudiated his wife Barbara. He refused and won the day. By 1550, when Sigismund summoned his second sejm, a reaction had begun in his favor, and the nobility was rebuked by Piotr Kmita, Marshal of the sejm, who accused them of attempting to unduly diminish the legislative prerogatives of the crown. The death of Queen Barbara, five months after her coronation (7 December 1550), under distressing circumstances, compelled Sigismund to contract a third, purely political union with his first cousin, the Austrian archduchess Catherine, also the sister of his first wife, Elisabeth, who had died within a year of her marriage to him, before his accession. Sigismund soon lost all hope of children by his third bride; he was the last male Jagiellon in the direct line so the dynasty was threatened with extinction. The King's marriage was a matter of great political import to Protestants and Catholics alike. The Polish Protestants hoped that he would divorce and remarry and thus bring about a breach with Rome at the very crisis of the religious struggle in Poland. He was not free to remarry until Queen Catherine's death on 28 February 1572, but he followed her to the grave less than six months later.

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