Esterka was a legendary Jewish mistress of Casimir the Great, the King of Poland between 1333 and 1370. She was the daughter of a poor tailor from Opoczno named Rafael.
Contemporary writers treated the love affair between Esterka and the Polish king as fact but modern historians have been more skeptical. The earliest written record of the story was made by Jan Dlugosz, less than a hundred years after the king's death. According to later writers the love affair took place while Casimir was married to Adelaide of Hesse and Krystyna Rokiczanka (the king had four wives during his reign). Supposedly the affair produced four children; two boys, Polka and Niemira, and two daughters (names not recorded). It is also suggested that, while the boys were raised according to the tenets of Casimir's Catholic faith, the girls retained their mother's Jewish faith. Another part of the story holds that the king had built a small castle in Bochotnica, north of Krakow, which he connected to Wawel Castle via an underground tunnel.
Several places, streets and monuments in Poland are named after Esterka, usually ones associated with her and the king. One such place is a remaining wall of an old castle build by Casimir in Kalisz where, according to local legend, Esterka's ghost comes at night to wait for her lover.
In some sources Esterka is presented as king's consort who actually lived with him at Wawel Castle although the two never married nor was she ever declared queen. Esterka is also sometimes credited by writers as being responsible for the expansion of privileges granted to Jews under Casimir.