Iago ab Idwal was a King of Gwynedd (reigned 950–979) and possibly Powys.

Iago was the son of the earlier King Idwal Foel but, upon Idwal's death in combat in 942, his uncle Hywel Dda invaded Gwynedd and seized the throne. On Hywel's death in 950, Iago and his brother Ieuaf were able to drive out their cousins at the Battle of Carno and reclaim the kingdom. Fighting continued, with the brothers raiding as far south as Dyfed in 952 and their cousins raiding as far north as the Conwy Valley in 954. The southern princes were finally defeated at the Battle of Llanrwst and chased back to Ceredigion.

Having won, the brothers then began to quarrel among themselves. Iago took Ieuaf prisoner in 969 and ruled another decade, with a brief hiatus in 974,^[why?] before Ieuaf's son Hywel usurped him in 979. There appears to be no surviving record of Iago's fate.

• Custennin ab Iago

• John Edward Lloyd (1911). A history of Wales: from the earliest times to the Edwardian conquest. Longmans, Green & Co.