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 that invaded the Anglo-Saxon kingdoms of England starting in 865. The Anglo-Saxon Chronicle describes him as a "heathen king".

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. Because Halfdan is not mentioned in any source that mentions Hvitserk, some scholars have suggested that they are the same individual – a possibility reinforced by the fact that Halfdan was a relatively common name among Vikings and Hvitserk "white shirt" may have been an epithet or nickname that distinguished Halfdan from other men by the same name.

Halfdan was the first Viking King of Northumbria and a pretender to the throne of the Kingdom of Dublin. 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. He died at the Battle of Strangford Lough in 877 trying to press his Irish claim.

Halfdan was one of the leaders of the Great Heathen Army that invaded the Anglo-Saxon kingdom of East Anglia in 865. According to the Norse sagas, this invasion was organised by the sons of Ragnar Lodbrok (Halfdan being one of them), to wreak revenge against Ælla of Northumbria. Ælla had supposedly had Ragnar executed in 865 by throwing him in a snake pit, but the historicity of this explanation is unknown. The invaders are usually identified as Danes, although the tenth-century churchman Asser stated that the invaders came "de Danubia", which translates as "from the Danube"; the fact that the Danube is located in what was known in Latin as Dacia suggests that Asser actually intended Dania, a Latin term for Denmark.

In the autumn of 865, the Great Heathen Army landed in East Anglia, where they remained over the winter and secured horses. 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. Late in 866, the army conquered the rich Northumbrian settlement of York. The following year, Ælla and Osberht made an alliance to retake the town. The attack was defeated, and both of them fell in the battle. 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.

Later in the year, the Army moved south and invaded the Kingdom of Mercia, capturing the town of Nottingham, where they spent the winter. The Mercian king, Burghred, responded by allying with the West Saxon King Æthelred, and with a combined force they laid siege to the town. The Anglo-Saxons were unable to recapture the city, but a truce was agreed whereby the Danes would withdraw to York. where they remained for over a year, gathering strength for further assaults.

The Danes returned to East Anglia in 869, this time intent on conquest. They seized Thetford with the intention of remaining there over the winter, but they were met by an East Anglian army. The East Anglian army was defeated and its commander, King Edmund, was slain. Medieval tradition identifies Edmund as a martyr who refused the Danes' demand to renounce Christ, and was killed for his steadfast Christianity. Ivar and Ubba are identified as the commanders of the Danes, and the killers of Edmund, and it is unknown what part, if any, Halfdan took.

Following the conquest of East Anglia, Ivar apparently left the Great Heathen Army – his name disappears from English records after 870. However, he is generally considered to be identical to Ímar, a Norse King of Dublin who died in 873. With Ivar in Ireland, Halfdan became the main commander of the Army, and in 870 he led it in an invasion of Wessex. Sometime after Ivar left the Army, a great number of Viking warriors arrived from Scandinavia, as part of the Great Summer Army, led by Bagsecg, bolstering the ranks of Halfdan's army. According to the Anglo-Saxon Chronicle, the Danes battled the West Saxons nine times, including at the Battle of Ashdown on 8 January 871. However, the West Saxons could not be defeated, and Halfdan accepted a truce from Alfred, newly crowned king of Wessex.