The Battle of Clontarf (Irish: Cath Chluain Tarbh) took place on 23 April 1014 at Clontarf, near Dublin, on the east coast of Ireland. It pitted an army led by Brian Boru, High King of Ireland, against a Norse-Irish alliance comprising the forces of Sigtrygg Silkbeard, King of Dublin; Máel Mórda mac Murchada, King of Leinster; and a Viking army from abroad led by Sigurd of Orkney and Brodir of Mann. It lasted from sunrise to sunset, and ended in a rout of the Viking and Leinster armies.

It is estimated that between 7,000 and 10,000 men were killed in the battle, including most of the leaders. Although Brian's forces were victorious, Brian himself was killed, as were his son Murchad and his grandson Toirdelbach. Leinster king Máel Mórda and Viking leaders Sigurd and Brodir were also slain. After the battle, the power of the Vikings and the Kingdom of Dublin was largely broken.

The battle was an important event in Irish history and is recorded in both Irish and Norse chronicles. In Ireland, the battle came to be seen as an event that freed the Irish from foreign domination, and Brian was hailed as a national hero. This view was especially popular during English rule in Ireland. Although the battle has come to be viewed in a more critical light, it still has a hold on the popular imagination.