Eystein Halfdansson (Old Norse: Eysteinn Hálfdansson) was the son of Halfdan Hvitbeinn of the House of Yngling according to Norse tradition. He inherited the throne of Romerike. Ari Thorgilsson in his Íslendingabók calls him Eystein the Swift (Old Norse: Eystein fret/fjert) without comment, in his king list, just naming his father and his son. Snorri does not call him by this nickname, but does give us a colorful story of his life.
According to Ynglingasaga, Eystein died returning from a viking raid to Varna, on the eastern side of the Oslofjord. Eystein's men had finished looting and pillaging the area and were already almost across the fjord, when King Skjöld of Varna, a great warlock, arrived at the beach and saw the sails of Eystein's ships. He waved his cloak and blew into it which caused a sailbearing spar (boom) of one close sailing ship in heavy sea to swing and hit Eystein so that he fell overboard and drowned. His body was salvaged and buried in a mound at Borre. Eystein was succeeded by his son Halfdan the Mild.
- Ari fróði Þorgilsson. "Islendingabok". Heimskringla.no. Retrieved 18 February 2016.