Saint Deiniol (died 584) was the first Bishop of Bangor in the Kingdom of Gwynedd, Wales. He is also venerated in Brittany as Saint Denoual. In English, the name is translated as Daniel but this is rarely used.
Very little is known of the saint's life, but the tradition that he was the first Bishop of Bangor is very strong. He was apparently consecrated in 545 by Saint David. The present Bangor Cathedral is dedicated to Deiniol and is said to be on the site where Deiniol's first monastery stood. His feast day is September 11.
A Latin life of Deiniol has been preserved in Peniarth MS226 transcribed by Sir Thomas Williams of Trefriw in 1602. He was the son of Dunod Fawr, son of Pabo Post Prydain. The family were originally rulers of an area in what is now the North of England, but having lost these were given lands by the king of Powys, Cyngen ap Cadell. Deiniol is said to have studied under Cadoc of Llancarfan and later was given land by Maelgwn Gwynedd king of Gwynedd to found a monastery on the site where Bangor Cathedral now stands. He attended the Synod of Llanddewi Brefi in c. 545 with Saint David when the subject of rules for penance was being discussed.
He may also have been the founder of the monastery of Bangor-on-Dee (Bangor Iscoed or Is-y-coed), Flintshire, though whether the early abbot there named Saint Dunod was his father or not is unclear. The church of Hawarden in Flintshire is dedicated to Deiniol. William Ewart Gladstone dedicated Saint Deiniol's Library, a library for arts students, in 1896. The church of Marchwiel is also dedicated to Deiniol and there are also dedications at Itton in Monmouthshire and Llangarran in Herefordshire. According to the Annales Cambriae, Deiniol died in 584 and was buried on Bardsey Island.
His name has been given to the Deiniol Centre, a shopping centre in Bangor.