Sefin was a semi-legendary 5th century female saint of Wales, and the grandmother of Saint David.
Nothing of her life is known except her place in the genealogies of some significant characters in the History of Wales. She was born 419AD in Cornwall, and was one of the daughters of King Brychan and Queen Prawst verch Tudwal. She was therefore a descendant of Roman Emperor Magnus Maximus, Saint Helen, Old King Cole, Saint Ursula, and Constantine the Great.
She married Cynyr Ceinfarfog and was the mother of two daughters, Saint Non and Gwen. Through Non she was the grandmother of Saint David, patron saint of Wales. Through Gwen she was the grandmother of Saint Cybi and Saint Fragan, who fathered Winwaloe, Wethenoc, Creirwy and Jacut.