Saint Asaph (or Asa) was, in the second half of the 6th century, the first Bishop of the Welsh See of that name.

No Welsh life of him is extant, but local tradition points out the site of his ash tree, his church, his well, and his valley, Onen Asa, Fynnon Asa, Llanasa, Pantasa. All these sites are near Holywell in Tegeingl (Flintshire), indicating probably that the saint once had hermitage in that neighbourhood. The Bonedd y Saint tells us that he was a son of King Sawyl Penuchel from the Old North or Y Gogledd Hen.

The want of a Welsh life, however, is in part compensated for by Jocelyn of Furness's life of St. Kentigern, alias Saint Mungo, the founder of the Diocese of Glasgow. This saint during his exile (c. 545) betook himself to Wales and there founded the Celtic Monastery of Llanelwy (the church on the River Elwy), as the Welsh still call the town of St Asaph. Of the building and government of few Celtic monasteries do we know so much as about Llanelwy. The church was built "of smoothed wood, after the fashion of the Britons, seeing that they could not yet build of stone". The 965 disciples, of whom Asa was one, were divided into three groups: 300 of the unlettered farmed the outlying lands, 300 worked in the offices around the monastery, and 365 (the number corresponds to the days of the year) attended to the divine services. Of these the oldest assisted Kentigern in the government of the diocese, and the rest were subdivided into three choirs. "As soon as one choir had terminated its service in church, immediately another entering commenced it: and that again being concluded another entered to celebrate." The founder, after the manner of other, used frequently to pray standing in the icy cold river, and once, having suffered very severely under this hardship, he sent the boy, Asaph, who was then attending him, to bring a fagot to burn and warm him. Asaph brought him live coals in his apron, and the miracle revealed to Kentigern the sanctity of his disciple. So when the old man was recalled to Strathclyde, after the Battle of Arfderydd, in 573 (the only definite date we have in the life), Asaph was consecrated bishop to succeed him, and became the first Welsh bishop of the see.

The feast of his deposition is kept on 1 May, but there are no further details of his life. Traditionally, he is said to have died in 601.