Świętosława was a Polish princess, daughter of Mieszko I of Poland and Dobrawa of Bohemia, and sister of Boleslaw I of Poland. She was married first to Eric the Victorious of Sweden and then Sweyn Forkbeard of Denmark, according to German chroniclers, and gave them the sons Olof, and Canute and Harald, respectively. The name is only known through an inscription that gives the name of Canute's sister and the assumption that this sister was named for her mother.
The Icelandic sagas give her rôle to Sigrid the Haughty. This account is not considered reliable by scholars such as Birgitta Fritz, who assign greater reliability to the contemporary chroniclers. Snorre Sturlasson also mentions a Slavic princess he calls Gunhild of Wenden.
Researcher Rafał T. Prinke in "The Identity of Mieszko I's Daughter and Her Scandinavian Relationships"(Roczniki Historyczne LXX (2004),[summary in German], Poznań – Warszawa 2004, ISBN 83-7063-429-X, pp. 81–110) establishes that Sigrid the Haughty (Sigríð Storråda) was the daughter of Skoglar Tosti, while the name Świętosława belonged to the mother of Mieszko I of Poland and his granddaughter by daughter Gunhilda (her mother being Mieszko I's second wife, Oda) and her husband, Sweyn Forkbeard. This would make Świętosława the sister of Canute the Great.
There is scant material in medieval chronicles to provide details regarding the marriages of Sweyn of Denmark and Erik of Sweden:
- Thietmar of Merseburg mentions that the daughter of Mieszko I of Poland and sister of Bolesław I Chrobry of Poland married Sweyn Forkbeard and gave him two sons, Cnut the Great and Harold II of Denmark, but he does not mention her name. Thietmar is probably the best informed of the medieval chroniclers addressing the question, since he was contemporary with the events described and well-informed about the events in Poland and Denmark. The assertion that Harald's and Canute's mother was Boleslaw's sister may explain some mysterious statements which appear in medieval chronicles, such as the involvement of Polish troops in invasions of England.
- Adam of Bremen writes almost a century later that a Polish princess—the sister or daughter of Bolesław I Chrobry of Poland—was the wife of Eric the Victorious and by this marriage the mother of Olof Skötkonung of Sweden, before she became mother of Cnut the Great and Harold II of Denmark in her second marriage with Sweyn. Adam's claims about the marriage to Eric are considered unreliable by many historians, since he is the only source to state this relationship and because he is writing several generations later. The scholia of Gesta Hammaburgensis Ecclesiae Pontificum mentions that it was the Polish king Boleslaw who gave the princess' hand in marriage. One problem is that Olof was born at latest in the early 980´s, before Boleslaw Chrobry came to power, and therefore was too old to be the unnamed princess´s son.
- Gesta Cnutonis regis mentions in one short passage that Canute and his brother went to the land of the Slavs, and brought back their mother, who was living there. This does not necessarily mean that his mother was Slavic, but nevertheless this chronicle strongly suggests that she was.
- There is an inscription in "Liber vitae of the New Minster and Hyde Abbey Winchester", that king Canute's sister's name was "Santslaue" ("Santslaue soror CNVTI regis nostri"), which without doubt is a Slavic name. J. Steenstrup suggests that Canute's sister may have been named after her mother, hence coining the (now generally agreed upon) hypothesis, that her Old Polish name is Świętosława, but only as a reconstruction based on a single mention of her daughter's name and the hypothesis that she named her daughter after herself.