Cadwallon ap Einion (c.460-534; reigned from c.500), also known as Cadwallon Lawhir ('Long Hand'), was a king of Gwynedd.

According to tradition, Cadwallon ruled during, or shortly after, the Battle of Mons Badonicus, and King Arthur's victory over the Saxons (depending on which date you believe, said battle accorded either sometime between the early 490s and the mid 510s). Although it is unlikely that Cadwallon himself was present at the event, he would likely have benefitted from the period of relative peace and prosperity throughout Britain that it augured. The most momentous military achievement of Cadwallon's reign was the final expulsion of Irish settlers on Ynys Mon (Anglesey), and the absorption of that island, which would later become the cultural and political base of the kingdom, into Gwynedd.

Cadwallon's second name, Lawhir, referred to an actual physical characteristic of the man: he apparently had unusually long arms! Iolo Goch claims that he could "reach a stone from the ground to kill a raven, without bending his back, because his arm was as long as his side to the ground."

According to Gildas, Cadwallon's son, Maelgwn, murdered his uncle to ascend to the throne, which suggests that someone other than Maelgwn himself inherited the kingdom upon Cadwallon's death. No clear evidence exists as to who this "lost king" might be (assuming, of course, that Gildas's account is reliable), but some have suggested the name of Owain Danwyn ('White-Tooth') as the unfortunate heir/victim.