The Battle of Mercredesburne was one of three battles fought as part of the conquest of what became the Kingdom of Sussex in southern England. The battles were fought between the Saxon leader Ælle's army and the local Britons.

The Kingdom of the South Saxons, today referred to as the Kingdom of Sussex, was one of the seven traditional kingdoms of the Heptarchy of Anglo-Saxon England. On the south coast of the island of Great Britain, it was originally a sixth-century Saxon colony and later an independent kingdom. The kingdom remains one of the least known of the Anglo-Saxon polities, with no surviving king-list, several local rulers and less centralisation than other Anglo-Saxon kingdoms. The South Saxons were ruled by the kings of Sussex until the country was annexed by Wessex, probably in 827, in the aftermath of the Battle of Ellendun. In 860 Sussex was ruled by the kings of Wessex, and by 927 all remaining Anglo-Saxon kingdoms were ruled by them as part of the new kingdom of England.