Barbara Radziwiłł (also known as Barbara Radziwiłłówna) (1520-1550/1551 (sources vary)), princess, queen of Poland, married to king Zygmunt II August.

Daughter of Lithuanian hetman Jerzy Radziwiłł.

She was married on 18 May 1537 to Stanislaus Gasztold , Palatin of Troki, who died on 18 December 1542.

Her romance and later marriage in 1547, Wilno, to king Zygmunt II August greatly increased the power of Radziwiłł family in Poland and Lithuania, as can be seen by the rise to power of Mikołaj 'Czarny' Radziwiłł and Mikołaj 'Rudy' Radziwiłł

There was substantial opposition towards her marriage with the king from many nobles (pl. szlachta), as the marriage was carried out with disregard to the laws governing royal marriages. It was pursued by the king who seemed to disregard political liabilities and consequences and be truly in love with Barbara. A divorce was demanded by Sejm, and many political games took place around the marriage issue, inflamed by machinations of king's mother Bona Sforza resulted in much humiliation and conflicts between king and many magnates. A deadlock lasting two years ensued. The opposition finally surrendered and Barbara was crowned queen in 1550 (or 1551, sources vary) on 3 May.

She died 8 May in Cracow, 5 days after the coronation. Her death was a severe loss to the king and there was unproven suspicion that she had been poisoned by Queen Bona.