Deiniol, Welsh for Daniel, (* about 510 in Wales; † in Wales about 584 ) was a bishop and saint

He was the son of a Celtic chieftain and founded in the early 6th century, two monasteries in Gwynedd. The first of these in Bangor, after King Maelgwn of Gwynedd he had transferred the appropriate country. Later he founded a monastery at Bangor -Is -y -Coed (Bangor upon Dee ), which was destroyed by 616 of Æthelfrith of Northumbria.

Deiniol took part in the Synod of Llanddewi Brefi, where he was consecrated by the Welsh patron saint St. David Bishop of Bangor. After his return from the Synod he should have founded several churches, such as in Llandeiniol in Dyfed and in Llanfor and Llanuwchllyn near Llyn Tegid. After his death, his remains were buried at Bardsey Iceland.

The Memorial of St. Deiniol is September 11.