Saint David (c. 512–587), (known in Welsh as Dewi Sant) was a Christian church official, later regarded as a saint and as the patron saint of Wales. David contrasts with such other national patron saints as England's Saint George, in that relatively much is known about his life.

David was a descendant of the royal house of Cunedda. Rhygyfarch wrote that David was the son of sanctus rex ceredigionis, where Sanctus has been interpreted as a proper name and its owner honoured by Welsh Christians as Saint Sant. The Latin phrase itself translates as "a holy king of Ceredigion". The king of Ceredigion in the 510s was Gwyddno Garanhir, according to regional tradition. His title Garanhir (crane legs), certainly indicated spiritual accomplishment to the Druids who bestowed it. As a son of King Gwyddno, David was a grandson of King Ceredig, and a nephew of King Maelgwn of Gwynedd, and a brother of Elphin the successor to the Kingdom of Ceredigion and the foster-father and first patron of the bard Taliesin.

He became renowned as a teacher and preacher, founding monastic settlements in Britain and Brittany, in a period when neighbouring tribal regions (that were to be united as England three hundred years later) were still mostly pagan. He rose to a bishopric, and presided over two synods, as well as going on pilgrimages to Jerusalem (where he was anointed as a bishop by the Patriarch) and Rome. St. David's Cathedral now stands on the site of the monastery he founded in a remote and inhospitable part of Pembrokeshire.

The Monastic Rule of David prescribed that monks had to pull the plough themselves without draught animals; to drink only water; to eat only bread with salt and herbs; and to spend the evenings in prayer, reading and writing. No personal possessions were allowed: to say "my book" was an offence. He taught his followers to refrain from eating meat or drinking alcohol. His symbol, also the symbol of Wales, is the leek.

The best-known miracle associated with St. David is said to have taken place on an occasion when he was preaching in the middle of a large crowd. When those at the back complained that they could not see or hear him, the ground on which he stood is reputed to have risen up to form a small hill so that everyone had a good view. The village which is said to stand on the spot today is known as Llanddewi Brefi. A more mundane version of this story is that he simply recommended that the synod participants move to the hilltop.

The document that contains much of the traditional tales about David is Buchedd Dewi, a hagiography written by Rhygyfarch in the 11th/12th century. One of Rhygyfarch's aims was that his document could establish some independence for the Welsh church, which was risking losing its independence following the Norman invasion of England in 1066. It is significant that David is said to have denounced Pelagianism during the incident before the ground rose beneath him.

William of Malmesbury recorded that David visited Glastonbury intending to dedicate the Abbey, as well as to donate a travelling altar including a great sapphire. He had a vision of Jesus who said that “ ‘‘the church had been dedicated long ago by Himself in honour of His Mother, and it was not seemly that it should be re-dedicated by human hands’’”. So David instead commissioned an extension to be built to the abbey, east of the Old Church. (The dimensions of this extension given by William were archaeologically verified in 1921.) One manuscript indicates that a sapphire altar was among the items King Henry VIII confiscated from the abbey at its dissolution a thousand years later. There are unverifiable indications that the sapphire may now be among the Crown Jewels.

His last words, according to the Buchedd Dewi, were "Be steadfast, brothers, and do the little things".

Unlike many contemporary "saints" recognised by the Welsh, David was actually canonised by Pope Callixtus II in 1120.

Saint David's Day is March 1.

David's life and teachings have also inspired a chorale work by Welsh composer Karl Jenkins, Dewi Sant . Inspired by the saint's teaching, it is a seven-movement that is best known for the classical crossover series Adiemus, who intersperses movements reflecting the themes of David's last sermon with those drawing from three Biblical Psalms.