Saint Seiriol was an early 6th century saint, who created a cell at Penmon on Anglesey, off Wales. He later moved to Puffin Island. He was a son of King Owain Danwyn of Rhôs.

According to legend, he and Saint Cybi were good friends, and would meet weekly near Llanerchymedd, at the Clorach wells. Saint Cybi would walk from Holyhead, facing the rising sun in the morning and setting sun in the evening. Saint Cybi was known as Cybi the Tanned, as he was tanned during his journey. Seiriol, travelling in the opposite direction, from Penmon, would have his back to the sun. Thus, he was known as Seiriol the Fair.

Seiriol was a younger brother of King Cynlas of Rhôs and King Einion of Lleyn. His cell at Penmon was rebuilt by his brothers, as they didn't think his humble residence was good enough. St Seiriol's Well lies in a small chamber adjoining its remains. Both are protected by Cadw, the publicly-funded body responsible for the historic monuments of Wales. Adjacent to them are the church and ruins of a monastery also dating back to Seiriol's day.

In his old age, Seiriol retired to Ynys Llanog which subsequently became known (in Welsh) as Ynys Seiriol. It was also known to the Vikings as Priestholm, but is now more commonly known as Puffin Island.