Lailoken was a Northern Brythonic prophet of the late 6th century. He is particularly associated with the Battle of Arfderydd in Cumberland (now Cumbria) and the area just to the north, over the border in modern Scotland. He apparently caused the battle and was subsequently sent mad with reproach.

Lailoken may be the original figure behind the legends of Myrddin Wyllt, in turn the prototype for the wizard Merlin of Arthurian legend. In a pre-12th century dialogue between Merlin and his sister, she refers to him as Lailoken, while the late-15th-century story Lailoken and Kentigern states that "some say he was called Merlynum".