Barbara Radziwiłł (Polish: Barbara Radziwiłłówna, Lithuanian: Barbora Radvilaitė; 6 December 1520/23 – 8 May 1551) was Queen of Poland and Grand Duchess of Lithuania as the second wife 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 Radziwiłł the Black and brother Mikołaj Radziwiłł the Red, 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.
Her contemporaries generally viewed Barbara in a negative light, accusing her of promiscuity and witchcraft. Her life became surrounded by many rumors and myths. She was a heroine of many legends in a wide range of literary works. From the 18th century, the life of Barbara became romanticized as the great tragic love affair. It has been used as an example of "love conquers all" with Bona Sforza often acting as the chief villain. It caught public imagination and has inspired many artists to create poems, plays, films, and other works. That made Barbara Radziwiłł one of the best known and most recognized women in the history of the Grand Duchy of Lithuania and Kingdom of Poland.