Saint Brynach was a 6th century Welsh saint. He is traditionally associated with Pembrokeshire, where several churches are dedicated to him.

A 12th century 'life' tells us that some time in the early 6th century, Brynach travelled (from where is unstated) to Rome and Brittany, and then on to Milford Haven. He erected various oratories near the rivers Cleddau, Gwaun, and Caman and at the foot of Carn Engyli (wrongly translated as 'Mountain of the Angels'), which was his most famous foundation. This monastery founded by Brynach was at present-day Nevern (in Welsh, Nanhyfer). The land was given to him by the local lord, Clether, who retired to Cornwall. Brynach was harassed by King Maelgwn of Gwynedd for a while, until he wrought miracles and the two came to terms. Saint Brynach died on 7 April, on which day his feast is celebrated. His church, overhanging the Nevern, is his lasting memorial. Details of Brynach's life seem to identify him with Saint Brannoc of Braunton in Devon, although his feast day is 7 January.

Brynach may be a form of the Irish name, Bernach. This has led to speculation that Saint Brynach came from Ireland. Iolo Morganwg, followed by Baring-Gould, supposed that he is the same as the chieftain 'Fernach' who came to Wales from Ireland with the young Brychan of Brycheiniog. However, Egerton Philimore rejects this identification. A 'Brennach Wyddel o'r Gogledd' or Brennach the Irishman of the North [of Britain] appears in the Welsh Triads. Rachel Bromwich does not believe Saint Brynach is meant and an Irish settler, perhaps from Galloway or Cumbria, is indicated.