Ieuaf ap Idwal (ruled 950 - 969) was a king of part of Gwynedd and possibly part of Powys (in what today is called Wales).
Ieuaf was the son of Idwal Foel. His real name was Idwal ab Idwal, but to prevent confusion with his father, he is referred to in the annals as Ieuaf (meaning Junior).
On his father's death in battle in 942 Ieuaf would have expected to succeed to the rule of Gwynedd together with his brother Iago ab Idwal. However Howell the Good King of Deheubarth took the opportunity to invade Gwynedd and disposses the young princes.
On Howell's death in 950 Ieuaf and Iago were able to claim the throne of Gwynedd, defeating the sons of Howell in battle at Nant Carno and driving them out of Gwynedd. Fighting continued between the two dynasties, with Iago and Ieuaf raiding as far south as Dyfed in 952 and the sons of Howell raiding as far north as the Conwy valley in 954 before being defeated in battle at Llanrwst and chased back to Ceredigion.
The sons of Idwal quarrelled and Iago took Ieuaf prisoner in 969, ruling Gwynedd alone until 979 when Ieuaf's son Hywel ap Ieuaf seized the throne from his uncle. According to J.E. Lloyd, Ieuaf remained in captivity until 988, which would imply that he was kept captive by his son.