Bleddyn ap Cynfyn, sometimes spelled Blethyn, was an 11th-century Welsh king. King Harold Godwinson and Tostig Godwinson installed him and his brother, Rhiwallon, as the co-rulers of Gwynedd on his father's death in 1063, during their destruction of the kingdom of their half-brother, king Gruffydd ap Llywelyn. Bleddyn became king of Powys and co-ruler of the Kingdom of Gwynedd with his brother Rhiwallon from 1063 to 1075. His descendants continued to rule Powys as the House of Mathrafal.

The Kingdom of Gwynedd was a Welsh kingdom and a Roman Empire successor state that emerged in sub-Roman Britain in the 5th century during the Anglo-Saxon settlement of Britain.