Halfdan Ragnarsson (; or Healfdene; ; died 877) was a Viking leader and a commander of the Great Heathen Army which invaded the Anglo-Saxon kingdoms of England, starting in 865. According to the tradition recorded in the Norse sagas he was one of the sons of Ragnar Lodbrok, and his brothers included Björn Ironside, Ivar the Boneless, Sigurd Snake-in-the-Eye and Ubba. He was the first King of Jórvík and also claimed the Kingdom of Dublin. He died at the Battle of Strangford Lough in 877 trying to press his Irish claim.