The Battle of Ashdown, on approximately 8 January 871, marked a significant West Saxon victory over a Danish Viking force in an unidentified location, possibly Kingstanding Hill in Berkshire or near Starveall close to Aldworth. Led by King Æthelred and his brother, Alfred the Great, against the Viking leaders Bagsecg and Halfdan, the battle is notably chronicled in the Anglo-Saxon Chronicle and Asser's Life of King Alfred.

Prelude to the battle saw the Vikings, having already conquered Northumbria and East Anglia by 870, advancing towards Wessex, reaching Reading around 28 December 870. Despite a West Saxon victory at Englefield led by Æthelwulf of Berkshire, a subsequent defeat at Reading set the stage for the confrontation at Ashdown.

During the battle, the Viking forces, advantageous in positioning atop a ridge, were met by the West Saxons who mirrored their divided formation. King Æthelred's late entry into the battle, following his Mass, and Alfred's preemptive attack were pivotal. The West Saxons' formation around a small thorn tree ultimately led to their victory, inflicting heavy losses on the Vikings, including the death of King Bagsecg and five earls. Despite this triumph, the victory was short-lived with subsequent defeats at Basing and Meretun, leading to King Æthelred's death and Alfred's succession post-Easter on 15 April 871.

The dating of the Battle of Ashdown is anchored to the death of Bishop Heahmund at Meretun on 22 March 871, placing Ashdown on 8 January, following a sequence of battles and Viking movements beginning from their arrival in Reading on 28 December 870. However, the exactness of these dates remains approximate due to potential inaccuracies in the chronology.