Illtud (also spelled Illtyd and, in corrupt English, Eltut, and, in Latin, Hildutus) (died mid-6th century), was a Welsh saint, founder and abbot of Llanilltud Fawr (Llantwit Major) in the Welsh county of Glamorgan.
The 7th century 'Life of Saint Samson' claims that Illtud was a disciple of Germanus of Auxerre (although this does not necessarily mean that he was taught by him directly), that he was the most learned Briton in the study of scripture and philosophy, and that he was the abbot of his monastery in Glamorgan.
The earliest Life of Illtud, full of implausible legends, was written about 1140. From it may perhaps be retained the claim that he sailed to Brittany with some corn ships to relieve the famine: some Breton churches and villages certainly bear his name. In the Life, Illtud is the son of a minor Breton prince named Bican Farchog, who begins his career as a skilled warrior, serving his maternal cousin, King Arthur, and others until his wild ways brought him into conflict with Saint Cadoc at Llancarfan Abbey. Illtud's warband raids the abbey, but the monks pursue them into a bog where the earth swallows all of them except Illtud. Cadoc reminds Illtud of his religion, and the humbled warrior takes up the monastic life.
In an age when any schooling was available only to a very few privileged people, perhaps Illtud's monastic school was the closest approximation in existence to an institution of higher education. Among Illtud's pupils were Saints Pol Aurelian, Samson of Dol, Gildas and David.
An inscription on a cross at says "Samson placed his Cross here for his soul, for the soul of Illtud, Samson, Rhain, Sawyl and Ebisar". It is possible that it was erected by Saint Samson himself in the 6th century, although it may be somewhat later. There is no formal evidence for a cult of Illtud surviving from before the 11th century. Many churches in Wales, however, are dedicated to him, while his monastery, reputed to contain hundreds of monks, was one of the most influential in South Wales. The Life tells of Illtud's bell being recovered from King Edgar's armies and of Illtud's protecting his people against the people of North Wales in the time of William the Conqueror. His feast day is November 6.