SIGISMUND II JAGELLON, King of Poland and Grand Duke of Lithuania. - Son of King Sigismund I and of Bona Sforza, b. on 1 Aug. 1520 at Kraków, d. on 7 July 1572 at Knyszyn.
Before his father's death (1548) he had already administered the Grand Duchy of Lithuania and shown an interest in the Protestant Reformation; he corresponded with Melanchthon and with Calvin, kept Protestant preachers at his court in Vilnius, and maintained cordial relations with the Lutheran sovereigns; but this understanding with the heretics was suddenly broken by the sovereign's secret marriage to Barbara Radziwill (1547), which was opposed by the Polish nobility of Protestant leanings, who demanded its annulment at the Diet of Piotrków in 1548: on the Polish side there was fear of an increased influence of the Lithuanian nobility over the government. With the support of the high dignitaries of the Catholic Church, S. nevertheless succeeded in breaking the opposition and in having Barbara crowned on 7 Dec. 1550 in the cathedral of Kraków, and he promised to grant particular aid to the Catholic Church, but in reality he tolerated Protestant preachers, including several Italians, spreading the ideas of the Reformation. In foreign policy he maintained good-neighborly relations with the Habsburgs and opposed Russian, Danish and Swedish attempts to occupy Livonia. With the Treaty of Lublin of 1 July 1569 he replaced the personal union between Poland and Lithuania with a real union, founded on the king and on the Seym (Diet of the Estates). - See pl. XXXVII.
BIBL. K. Völker, Kirchengesch. Polens, Berlin-Leipzig 1930, p. 149 ff. with bibl.; D. Cantimori, Eretici ital. del Cinquecento, Florence 1939, passim; J. Pajewski, Zygmunt August and the Union of Lublin 1548-72, in Cambridge History of Poland, I, Cambridge 1950, pp. 348-68.