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Sigurd Hart or Sigurd Hjort was a legendary king of Ringerike (modern central south Norway), during the late 9th or early 10th centuries. he is mentioned in Ragnarssona þáttr ("The Tale of Ragnar's Sons") and in Halvdan Svartes saga ("Halfdan the Black's Saga").
Ragnarssona þáttr states that Sigurd Hart was the son of Helgi the Sharp (the great-great-grandson of king Ring of Ringerike) of the Dagling dynasty and Helgi's wife Aslaug. Helgi was reportedly the son-in-law of Sigurd Snake-in-the-Eye (one of Ragnar Lodbrok's sons) and Blaeja, the daughter of king Aelle II of Northumbria.
Traditional sources state that Sigurd Hart was only 12 years old, he slew a berserker named Hildibrand in a duel, and 11 other men. He married a woman named Ingeborg (supposedly the daughter of the historical Jutish chieftain Harald Klak, c. 785 – c. 852, although Harald was probably too old for that to be true). Sigurd Hart and Ingeborg had children named Guttorm Sigurdsson and Ragnhild Sigurdsdotter. When Sigurd Hart's uncle, king Fróði of Ringerike died, Sigurd Hart supposedly went to Norway to succeed him as king of Ringerike.
- One is the suggestion that he was the father of Ragnhild Sigurdsdotter,  who was kidnapped by, and the wife of, Halfdan the Black (c. 810 – c. 860), and mother of Harald Fairhair.. This would seem impossible unless two different men named Sigurd Hart were involved – given that most sources suggest that his father was active in the late 9th century, which would make Sigurd Hart about two generations younger than Ragnhild Sigurdsdotter.
- Another unlikely claim suggests that the legendary Danish king Harthacanute (born c. 880) was a descendant of Sigurd Hart, although Harthacanute may well have been born before Sigurd. (Harthacanute succeeded Sigtrygg Gnupasson as the king of Zealand, Scania and Halland, but he lost Viken (Oslofjord). He was the father of Gorm the Old (born before 900), the king of Denmark. Gorm succeeded his father as king and married Thyra.  Gorm's son, Harald Bluetooth (born c. 935) succeeded his father as king and married Gyrid of Sweden. They had a son named Sweyn Forkbeard. Sweyn succeeded his father as king and married Gunhild (Świętosława of Poland). They had a son named Cnut the Great. Sweyn also ruled England in his lifetime and established the Danish Empire. When Sweyn died, his elder son Harald Svendsen became the King of Denmark, while England's former king, Ethelred reclaimed the throne. Following Harald's death, his brother Cnut the Great became king, re-established the Danish North Sea Empire. He married Emma of Normandy with whom he had a son named Harthacnut. When Cnut died, Harthacnut became king of Denmark and England. Upon his death, Edward the Confessor became ruler of England in 1042.)
- Halfdan the Black Archived 2011-07-22 at the Wayback Machine (koti.kontu.la)
- "The Tale of Ragnar's Sons", chapter 4. Translation by Peter Tunstall
- "The Tale of Ragnar's Sons", chapter 3. Translation by Peter Tunstall
- translated by Peter Tunstall. "Chapter 5. The Fall of Sigurd Hart". The Saga of Ragnar Lodrok and his Sons. Retrieved December 15, 2016.
- The Ragnarssona þáttr and Heimskringla relate that a berserker from Hadeland named Haki (Hake) killed Sigurd Hart, but lost a hand in the fight. Haki then went to Sigurd Hart's residence at Stein and took Sigurd's children Ragnhild and Guttorm. Haki returned with the children and all the loot to Hadeland. Before Haki (Hake) recuperated from his wounds and could marry the 15-year-old Ragnhild, she was stolen for a second time, by Halfdan the Black.
- Some sources claim that Thyra was a daughter of Harald Klak (c. 785 – c. 852); as with his other purported ties to Sigurd Snake-in-the-Eye, however Harald Klak was too old for that to be true. There are also other, more credible accounts of Thyra's parentage.
- Lund, Niels. "Gorm den Gamle". Den Store Danske, Gyldendal (in Danish). Retrieved December 15, 2016.
- Lunga, Peter. "Hardeknud – av England og Danmark" [Harthacnut - of England and Denmark]. Store norske leksikon (in Norwegian). Retrieved December 15, 2016.