Ongentheow (Old English: Ongenþeow, Ongenþio, Ongendþeow; Swedish: Angantyr) (died ca. 515) was the name of a semi-legendary Swedish king of the house of Scylfings, who appears in Old English sources.
He is generally identified with the Swedish king Egil Vendelcrow mentioned in Ynglingatal, Historia Norwegiae and in Ynglinga saga. The reason why they are thought to have been the same is that they have the same position in the line of Swedish kings and are described as the fathers of Ohthere and grandfathers of Eadgils.
The name Ongentheow contains as its second element "servant, slave". The first appears to be ongēan "against, opposite".
In the Old English epic Beowulf Ongentheow is described as a fearsome warrior and it took two warriors Eofor and Wulf Wonreding to take him down.
The epic tells that the Geats under their new king Hæþcyn captured the Swedish queen, but old king Ongenþeow saved her, at a hill fort called Hrefnesholt, although they lost her gold. Ongentheow killed Hæþcyn, and besieged the Geats at Hrefnesholt. The Geats were, however, rescued by Hygelac, Hæþcyn's brother, who arrived the next day with reinforcements. Having lost the battle, but rescued his queen, Ongenþeow and his warriors returned home.
However, the war was not over. Hygelac, the new king of the Geats, attacked the Swedes. The Geatish warriors Eofor and Wulf fought together against the hoary king Ongenþeow. Wulf hit Ongentheow's head with his sword so that the old king bled over his hair, but the king hit back and wounded Wulf. Then, Eofor retaliated by cutting through the Swedish king's shield and through his helmet, giving Ongentheow a death-blow. Eofor took the Swedish king's helmet, sword and breastplate and carried them to Hygelac. When they came home, Eofor and Wulf were richly awarded, and Eofor was given Hygelac's daughter. Because of this battle, Hygelac is referred to as Ongentheow's slayer.