Saint Deiniol (died 572) was traditionally the first Bishop of Bangor in the Kingdom of Gwynedd, Wales.

The present Bangor Cathedral, dedicated to Deiniol, is said to be on the site where his monastery stood.

According to a Latin Life of Deiniol, preserved in Peniarth MS226 and transcribed in 1602 by Sir Thomas Williams of Trefriw, he was the son of Abbot Dunod Fawr, son of Pabo Post Prydain.

Sir David Trevor describes Deiniol as one of the seven blessed cousins who had spent part of his early life as a hermit "on the arm of Pembrokeshire" but was called to be a bishop despite deficiencies in his formal education.

He attended the Synod of Llanddewi Brefi in c. 545 with Saint David when the subject of rules for penance was being discussed.