Ketill Björnsson, nicknamed Flatnose (Old Norse: Flatnefr), was a Norse King of the Isles of the 9th century.
The story of Ketill and his daughter Auðr (or Aud) was probably first recorded by the Icelander Ari Þorgilsson (1067 – 1148). Ari was born not long after the death of his great-grandmother Guðrún Ósvífrsdóttir – a prominent character in the Laxdæla saga whose husband, Thorkell Eyjolfsson, was descended from Auðr. Ari was thus a direct descendant of Ketill and so, when he wrote his story of Ketill, he was drawing in part on oral traditions amongst his own relatives.
Ketill was also depicted in such works as the Laxdæla saga, Eyrbyggja saga and the Saga of Erik the Red, while his genealogy was described in detail in the Landnámabók.
However, like many other medieval histories, all of these Old Norse works were written long after the events they described. No contemporaneous records of Ketill's life are known to exist, with the arguable exception of a single entry in the Annals of Ulster.
Ketill Björnsson was the son of Björn Grímsson. In the Laxdaela saga he is recorded as being from Romsdal (Raumsdal), a valley in the county of Møre og Romsdal, between Nordmøre and Sunnmøre and from Sogn in the Landnámabók.
After Harald Fairhair had won the decisive Battle of Hafrsfjord in the late 9th century, many fled from Norway. According to the Orkneyinga saga, some of these Vikings began to raid Norway in summer from the Orkney and Shetland islands north of mainland Scotland. For this reason Harald set sail to uproot the attackers. He defeated them and also took possession of the Hebrides and the Isle of Man. This story is retold in the Eyrbyggja saga, but here it is Ketill rather than Harald who led the expedition, and after the initial victory the former retained the islands as "personal domain" rather than bringing them under Harald's rule. In the Laxdaela saga the same story is told, but here Ketill is one of the Vikings who have fled to the Isles to escape Harald's tyranny. In the Landnámabók the initial conquest is led by Harald, but as soon as he returned to Norway the raiders regrouped. At this point Harald sent Ketill to win the islands back again. Ketill did so but paid no tribute, at which point Harald took possession of what was owed from possessions of Ketill in Norway and sent away Ketill's sons. Some sources refer to Ketill as "King of the Sudreys" although there is little evidence that he himself claimed that title.