Gwrgan Fawr (meaning Gwrgan the Great; also, in Latin, Gurgantius; English Fergus; died c. 645) was a king of Ergyng, a south-east Welsh kingdom of the Early Middle Ages. He was the son of Cynfyn and rightful heir to the Ergyng throne which, however, was initially usurped by Gwrfoddw around AD 615. Gwrgan probably fled Ergyng when his inheritance was seized by Gwrfoddw Hen. He appears in four charters in the Book of Llandaff during the episcopate of Bishops Euddogwy and Inabwy. Later, Gwrgan features as king in charters in the Book of Llandaff associated with Bishops Oudoceus and .