Sigurd Syr (Old Norse: Sigurðr Sýr) (died 1018) was a Norwegian petty king of Ringerike, a region in Buskerud. By his marriage with Åsta Gudbrandsdatter, he was the father of King Harald Hardrada and stepfather of King Olaf the Saint. His nickname "Syr" can be translated as meaning "sow". The reason for this is unknown; P.A. Munch suggested that it was because his attention was so focused on farming that he "rooted the ground like a pig", but it could equally be for some other reason.
The traditional view of Sigurd 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 Hrise. However, many modern historians regard the Fairhair genealogy as in large part invented. Sigurd's father Halfdan may not have been Halfdan of Hadafylke, Harald's grandson; there are no contemporary attestations of such a Halfdan. Doubt has also been cast on Sigurd Rise, a relatively obscure son of Harald Fairhair by a Sami girl named Snæfrid.
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 was not fond of the pomp and ceremony of nobility, but played his expected part.
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 ambitious, but 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 for a new war against Sweden, Denmark and England that commenced in 1015. This war was his stepson's cause.