Barbara Radziwiłł was Queen of Poland and Grand Duchess of Lithuania as consort of Sigismund II Augustus, the last male monarch of the Jagiellon dynasty. Barbara, a great beauty and already widowed, became a royal mistress most likely in 1543 and they married in secret in July or August 1547. The marriage caused a scandal; it was vehemently opposed by Polish nobles, including Queen mother Bona Sforza. Sigismund Augustus, assisted by Barbara's cousin Mikołaj "the Black" Radziwiłł and brother Mikołaj "the Red" Radziwiłł, worked tirelessly to gain recognition of their marriage and to crown Barbara as Queen of Poland. They succeeded and Barbara's coronation was held on 7 December 1550 at Wawel Cathedral. However, her health was already failing and she died just five months later. Even though it was brief, her reign propelled the Radziwiłł family to new heights of political power and influence.

Sigismund II Augustus was King of Poland and Grand Duke of Lithuania, the son of Sigismund I the Old, whom Sigismund II succeeded in 1548. He was the first ruler of the Polish–Lithuanian Commonwealth and the last male monarch from the Jagiellonian dynasty.