Guithelin was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He became king after the death of Gurguit Barbtruc^[1] and reigned approximately 379BC.

He ruled liberally and temperately for his life. His Queen consort was an artisan and noblewoman named Marcia; when he died, his wife took over the government as regent for their son, Sisillius II.^[2]