Bleddyn ap Cynfyn (died 1075) was a Prince of the Welsh Kingdoms of Gwynedd and of Powys.
Bleddyn was the son of Princess Angharad ferch Maredudd (of the Dinefwr dynasty of Deheubarth) with her second husband Cynfyn ap Gwerstan, a Powys Lord, about whom little is now known. He may have been son of an English Saxon - the name has been postulated as being derived from Werestan.
His mother Angharad was previously widow of Llywelyn ap Seisyll and also mother of Gruffudd ap Llywelyn.
Bleddyn was married to Hear of Powys.
When Gruffydd ap Llywelyn was killed by his own men after being defeated by the Saxon Harold Godwinson in 1063, his realm was divided among several Welsh Princes. Bleddyn and his brother Rhiwallon ap Cynfyn, as half brothers to Gruffudd succeeded to his lands but first as vassals and allies of the Saxon King of England, Edward the Confessor and then submitted to Harold and from him received Gwynedd and Powys.
They continued Gruffudd's policy of allying to the Mercian Saxons to resist the threat from William the Conqueror.
In 1067 Bleddyn and Rhiwallon joined with the Mercian Eadric the Wild in an attack on the Normans at Hereford, ravaged the lands as far as the River Lugg then in 1068 allied with Earl Edwin of Mercia and Earl Morcar of Northumbria in another attack on the Normans.
Bleddyn was challenged by the two sons of Gruffydd ap Llywelyn, but defeated them at the battle of Mechain in 1070, one being killed and the other dying of exposure after the battle. Bleddyn's brother Rhiwallon was also killed in this battle, Bleddyn emerging as the only one of the four to survive the bloody encounter and he ruled Gwynedd and Powys alone until his death.
In 1073 Robert of Rhuddlan stealthily established his forces on the banks of the River Clwyd and attempted to ambush and capture Bleddyn, narrowly failing but seizing valuable booty from the raids further south.
He was killed in 1075 by Rhys ab Owain of Deheubarth and the nobility of Ystrad Tywi in South Wales, a killing which caused much shock throughout Wales.
When Rhys ab Owain was defeated in arms at the Battle of Goodwick and forced to become a fugitive by Bleddyn's cousin and successor as King of Gwynedd, Trahaearn ap Caradog in 1078 and killed by Caradog ap Gruffydd of Gwent shortly afterwards, this was hailed as "vengeance for the blood of Bleddyn ap Cynfyn".
Bleddyn is said in the Brut y Tywysogion to have been a benevolent ruler: "the most lovable and the most merciful of all kings ... he was civil to his relatives, generous to the poor, merciful to pilgrims and orphans and widows and a defender of the weak ...". and "the mildest and most clement of kings" and he "did injury to none, save when insulted.... openhanded to all, terrible in war, but in peace beloved."
He was responsible for a revision of Welsh law in the version used in Gwynedd. After his death Gwynedd was seized by Trahaearn ap Caradog and later recovered for the line of Rhodri the Great by Gruffydd ap Cynan, but in Powys Bleddyn was the founder of a dynasty which lasted until the end of the 13th century.