Sigismund II Augustus II (Polish: Zygmunt II August, Ruthenian: Żygimont III Awgust I, Belarusian: Жыгімонт Аўгуст; Lithuanian: Žygimantas III Augustas I, German: Sigismund II.

August) (1 August 1520 – 7 July 1572) is King of Poland and Grand Duke of Lithuania, the only son of Sigismund I the Old, whom Sigismund II succeeds in 1548.

Married three times, the last of the Jagiellons remains childless, and thus the Union of Lublin introduces Elective monarchy.