Halfdan Ragnarsson was a Viking leader and a commander of the Great Heathen Army which invaded the Anglo-Saxon kingdoms of England, starting in 865.

The Great Heathen Army, also known as the Viking Great Army, was a coalition of Scandinavian warriors who invaded England in AD 865. Since the late 8th century, the Vikings had been engaging in raids on centres of wealth, such as monasteries. The Great Heathen Army was much larger and aimed to conquer and occupy the four kingdoms of East Anglia, Northumbria, Mercia and Wessex.