Sigurd I Magnusson (c. 1090 – 26 March 1130), also known as Sigurd the Crusader (Old Norse: Sigurðr Jórsalafari, Norwegian: Sigurd Jorsalfar), was King of Norway from 1103 to 1130. His rule, together with his half-brother Øystein (until Øystein died in 1123), has been regarded by historians as a golden age for the medieval Kingdom of Norway. He is otherwise famous for leading the Norwegian Crusade (1107–1110), earning the eponym "the Crusader".

Sigurd was one of the three sons to King Magnus III, the other two being Øystein and Olaf. They were all illegitimate sons of the king with different mothers. To avoid feuds or war, the three half-brothers co-ruled the kingdom from 1103. Sigurd would rule alone after Olaf died in 1115 and Øystein in 1123.

Before being proclaimed King of Norway, Sigurd was also styled as King of the Isles and Earl of Orkney. He would pass the Earl of Orkney title on to Haakon Paulsson, a son of Paul Thorfinnsson.

Many historians have viewed Sigurd and Øystein's rule as a golden age for the medieval Kingdom of Norway. The country was able to flourish both in wealth and expansion, Sigurd's participation in the crusades as well as gained international recognition and prestige.

In 1098, Sigurd accompanied his father, King Magnus III, on his expedition to the Orkney Islands, Hebrides and the Irish Sea. He was made Earl of Orkney the same year, following the swift removal of the incumbent Earls of Orkney, Paul and Erlend Thorfinnsson. He was also apparently, made King of the Isles in that same year, following the overthrow of their king by his father, Magnus. Although Magnus was not directly responsible for the death of the previous King of the Isles, he was the next ruler of the kingdom, most likely due to his conquest of the islands. This was the first time the kingdom had been under direct control by a Norwegian king. It is not certain whether Sigurd returned home with his father to Norway after the 1098 expedition. However, it is known that he was in Orkney when Magnus returned west in 1102 for his next expedition. While there, a marriage alliance was negotiated between Magnus and Muircheartach Ua Briain. He was High King of Ireland, one of the most powerful rulers in Ireland, as well as the ruler of Dublin. Sigurd was to marry Muirchertach's daughter Bjaðmunjo, a young Irish princess and for a short period, Queen consort.

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