Dyfnwal I was ruler of the
Kingdom of Strathclyde for some period in the early tenth century, and perhaps the late ninth century. He is known only from the
Chronicle of the Kings of Alba, which states that "in this time Doneualdus king of the Britons died". "This time" refers to the reign of king
Causantín mac Áeda after 908, but before 916. Dyfnwal was presumably a relative of
Eochaid, either through the dynasty of
Kenneth MacAlpin (Cináed mac Ailpín) or the native British dynasty of
Run, but nothing of this is known for certain. However, the fact that the
Chronicle of the Kings of Alba reports his death, rather than anything else, means we known for certain he was different from
Dyfnwal II, a later Strathclyde king.