Sigurd Syr (Old Norse: Sigurðr Sýr) (died ca. 1018) was a Norwegian petty king of Ringerike, a region in Buskerud. He was notable in Norwegian history largely through his association with Kings Harald Hardrada and Olaf II of Norway. By his marriage with Åsta Gudbrandsdatter after her first husband Harald Grenske had died, Sigurd Syr was stepfather of King Olaf and the father of King Harald. 
The traditional view of Sigurd Halvdansson Syr's pedigree, as presented in various Icelandic poems and historical sagas culminating in Snorri Sturluson's Heimskringla, is that he was a great-grandson of King Harald Fairhair, through Harald's son Sigurd Rise. Doubt has been cast on his connection with Sigurd Rise, a relatively obscure son of Harald Fairhair by a Sami girl named Snæfrid. Sigurd's father Halfdan may not have been King Harald's grandson Halfdan of Hadafylke.  There are no contemporary attestations of such a Halfdan. It is not very likely that this lineage represent historical realities. Many modern historians regard the Fairhair genealogy as in large part invented.
According to Heimskringla, Sigurd Syr was a prudent man, taciturn and generally modest, although very wealthy. He was known as a good caretaker of his property, personally involved in managing it, and wise and peaceful. He may well have owned several large farms. His nickname "Syr" can be translated as meaning "sow or swine" (related to Old Norse svīn). The reason for this is unknown; P.A. Munch suggested that it was because his attention was so focused on farming and that his nickname meant: "One who digs or roots in the soil of the field". Since Sigurd was a respected man of high birth, the epithet was most probably an honorable title.  
Sigurd Syr was baptized into the Christian faith in 998. According to the Heimskringla, "King Olaf Tryggvason came to Ringerike to spread Christianity, Sigurd Syr and his wife allowed themselves to be baptized".
Sigurd Syr was not personally politically ambitious. He was not fond of the pomp and ceremony of nobility, but played his expected part. He fully supported his stepson Olaf's ambitions to leadership and command. In 1014, he successfully petitioned his peers, the other Norwegian local kings, for their support in his stepson's cause when Olav wanted to build an army in Norway. 
- "Sigurd Syra". Lokalhistoriewiki. Retrieved October 27, 2015.
- Snorri Sturluson, Heimskringla: History of the Kings of Norway, tr. Lee M. Hollander, The American-Scandinavian Foundation, Austin: University of Texas Press, 1964, repr. 1992, ISBN 0-292-73061-6, Saga of Óláf Tryggvason ch. 60, p. 200.
- M. Sjöström, "Scandinavian medieval descendants of Charlemagne: A detailed genealogy of the issue of Agnes Haakonsdottir, of the so-called Fairhair dynasty", Foundations - Journal of the Foundation for Medieval Genealogy vol 2 (2007:4, July), pp. 253-76: "It is very likely that the lord Halvdan, father of kinglet Sigurd Syr, was not identical with a possible Halvdan in Hadafylke, grandson of king Harald".
- Knut Helle, Cambridge History of Scandinavia, Volume I, Prehistory to 1520, Cambridge University Press, 2003, ISBN 0-521-47299-7, p. 191.
- See Joan Turville-Petre, "The Genealogist and History: Ari to Snorri", Saga-Book 20 (1978-81), pp. 7-23 (pdf), especially p. 15: "[Harald Fairhair's] successors Óláfr Tryggvason, Óláfr helgi and the Norwegian kings descended from Haraldr harðráði [e]ach . . . represented a separate line, which had to be connected to Haraldr: as he was their political forerunner, so also he must be their common ancestor".
- Heimskringla ed. Hollander, St. Óláf's Saga ch. 33, p. 268.
- "Etymology of the Norse word svin". Online Etymology. Retrieved October 28, 2015.
- Claus Krag, "Sigurd Halvdansson Syr – utdypning", Store norske leksikon (Norwegian)
- Wikisource text King Olaf Trygvason's Saga, Part II, "Section 24, Olaf Haraldson Baptized"
- Sigwart Sörensen, Norway, New York: Collier, 1899, OCLC 474863147, pp. 79-80.