Siward ( or more recently ) or Sigurd (Old English: Sigeweard, Old Norse: Sigurðr digri) was an important earl of 11th-century northern England. The Old Norse nickname Digri and its Latin translation Grossus ("the stout") are given to him by near-contemporary texts. It is possible Siward may have been of Scandinavian or Anglo-Scandinavian origin, perhaps a relative of Earl Ulf, although this is speculative and unclear. He emerged as a powerful regional strongman in England during the reign of Cnut ("Canute the Great", 1016–1035). Cnut was a Scandinavian ruler who conquered England in the 1010s, and Siward was one of the many Scandinavians who came to England in the aftermath of that conquest. Siward subsequently rose to become sub-ruler of most of northern England. From 1033 at the latest Siward was in control of southern Northumbria, that is, present-day Yorkshire, governing as earl on Cnut's behalf.

He entrenched his position in northern England by marrying Ælfflæd, the daughter of Ealdred, Earl of Bamburgh. After killing Ealdred's successor Eadulf in 1041, Siward gained control of all Northumbria. He exerted his power in support of Cnut's successors, kings Harthacnut and Edward, assisting them with vital military aid and counsel. He probably gained control of the middle shires of Northampton and Huntingdon by the 1050s, and there is some evidence that he spread Northumbrian control into Cumberland. In the early 1050s Earl Siward turned against the Scottish king Mac Bethad mac Findlaích ("Macbeth"). Despite the death of his son Osbjorn, Siward defeated Mac Bethad in battle in 1054. More than half a millennium later the adventure in Scotland earned him a place in William Shakespeare's Macbeth. Siward died in 1055, leaving one son, Waltheof, who would eventually succeed to Northumbria. St Olave's church in York and nearby Heslington Hill are associated with Siward.