Edmund Ironside was King of the English from 23 April to 30 November 1016. He was the son of King Æthelred the Unready and his first wife, Ælfgifu of York. Edmund's reign was marred by a war he had inherited from his father; his cognomen "Ironside" was given to him "because of his valour" in resisting the Danish invasion led by Cnut.

In summer of 1013 Sweyn Forkbeard launched a full-scale invasion of England, driving out Aethelred by the end of the year. Edmund and his elder brother Aethelstan did not follow their father in exile. Sweyn died unexpectedly in February of 1014, and Æthelred was able to quickly reclaim the throne, driving out Sweyn's son Cnut, whom the Danes elected king. Aethelstan had died by June of 1014, making Edmund heir apparent.

After regaining the throne, the royal family set about strengthening its hold on the country with the assistance of Eadric Streona (Edmund's brother-in-law). People who had sided with the Danes were punished, and some were killed. In one case, two brothers, Morcar and Sigeferth, were killed and their possessions were taken by Æthelred. Sigeferth's widow Ealdgyth was imprisoned within a monastery, but in summer of 1015 Edmund unofficially named himself the Earl of the East Midlands and raised a revolt against his father. Without the king's permission he took Ealdgyth from the monastery, and married her; it would have been a politically advantageous marriage, since she was a member of one of the strongest families in the Midlands. Cnut returned to England in August 1015. Over the next few months, Cnut pillaged most of England. After unsuccessfully attempting to fight the Danes in the north, Edmund joined Æthelred to defend London.

Æthelred died on 23 April 1016, the nobility and citizens present at London elected Edmund king, while the rest of the English nobility declared their allegiance to Cnut. It was not until the summer of 1016 that any serious fighting was done: Edmund fought five battles against the Danes and their supporters, ending in his defeat on 18 October at the Battle of Assandun, after which they agreed to divide the kingdom, Edmund taking Wessex and Cnut the rest of the country. Edmund died shortly afterwards on 30 November, leaving two sons, Edward and Edmund; however, Cnut became king of all England, and exiled the remaining members of Edmund's family.