Sigismund II Augustus I (, Ruthenian: Zygimont III Awgust I, , ) (1 August 1520 – 7 July 1572) was 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 an elective monarchy.