Cadwallon ap Cadfan (died 634) is the King of Gwynedd from around 625 until his death in battle.

The son and successor of Cadfan ap Iago, he is best remembered as the King of the Britons who invaded and conquered Northumbria, defeating and killing its king, Edwin, prior to his own death in battle against Oswald of Bernicia.

His conquest of Northumbria, which he holds for a year or two after Edwin dies, make him the last Briton to hold substantial territory in eastern Britain until the rise of the Tudor dynasty.

He is thereafter remembered as a national hero by the Britons and as a tyrant by the Anglo-Saxons of Northumbria.