Bagsecg, Viking leader
Bagsecg (born: 830-died: 8 January, 871), also known as Bacgsecg, was a viking and a leader of the Great Army, which invaded England. According to the Anglo-Saxon Chronicle, Bagsecg and Healfdene (Norse Hálfdan) were joint commanders of the Great Army that invaded the Kingdom of Wessex during the northern winter of 870/71.

The Great Army is recorded as setting up camp at Reading and fighting the forces of Æthelred, King of Wessex, in several engagements during 871. At one of these, the Battle of Ashdown, Bagsecg and five Viking earls were killed.

After Bagsecg's death, Healfdene seems to have become the sole leader of the Great Army. He was the principal leader when the vikings overwintered in London in 871/72. In addition, three other viking kings had emerged by 875 (according to the Anglo-Saxon Chronicle); some of these may have been elevated to kingly status as a consequence of Bagsecg's demise.

During the 18th and 19th centuries, Bagsecg was erroneously associated with Wayland's Smithy, a neolithic long barrow in south Oxfordshire. A folk legend arose that the barrow was his tomb or a memorial to Bagsecg. Likewise, other prehistoric sites in south central England, such as the Seven Barrows, have been erroneously regarded as memorials to those who fell at the Battle of Ashdown.