Halfdan Ragnarsson (Old Norse: Hálfdan; Old English: Halfdene or Healfdene; Old Irish: Albann; 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.

Halfdan was one of six sons of Ragnar Lodbrok named in Norse sagas; his brothers and half-brothers included Björn Ironside, Ivar the Boneless, Sigurd Snake-in-the-Eye, Ubba and Hvitserk.

It is also possible he was for a time co-ruler of Denmark with his brother Sigurd Snake-in-the-eye, because Frankish sources mention a certain Sigfred and Halfdan as rulers in 873.

[7] The following year the army moved northwards and invaded Northumbria, which was at that time in the middle of a civil war between Ælla and Osberht, opposing claimants for the Northumbrian throne.

[8] With no obvious leader, Northumbrian resistance was crushed and the Danes installed a puppet-king, Ecgberht, to rule in their name and collect taxes for them.

[10] Later in the year the Army moved south and invaded the Kingdom of Mercia, capturing the town of Nottingham, where they spent the winter.

[13] Medieval tradition identifies Edmund as a martyr who refused the Danes' demand to renounce Christ, and was killed for his steadfast Christianity.

[15] Following the conquest of East Anglia Ivar apparently left the Great Heathen Army – his name disappears from English records after 870.

[18] According to the Annals of Ulster, Eystein Olafsson, King of Dublin was "deceitfully" killed in 875 by "Albann", a figure generally agreed to be Halfdan.

According to Hilda Ellis Davidson, writing in 1979, "certain scholars in recent years have come to accept at least part of Ragnar's story as based on historical fact".