Brychan Brycheiniog was a legendary 5th-century king of Brycheiniog (Brecknockshire alternatively Breconshire ) in South Wales.

Celtic hagiography tells us that Brychan was born in Ireland, the son of a Prince Anlach, son of Coronac, and his wife, Marchel, heiress of the Welsh kingdom of Garthmadrun (Brycheiniog), which the couple later inherited. Upon his father's death, he returned to Garthmadrun and changed its name to Brycheiniog. Brychan's name may be a Welsh version of the Irish name Broccán and that of his grandfather Coronac may represent Cormac. The Life of St. Cadoc by Lifris (c. 1100) portrays Brychan fighting Arthur, Cai and Bedivere because of King Gwynllyw of Gwynllwg's abduction of his daughter St. Gwladys from his court in Talgarth.

He is occasionally described as an undocumented saint but the traditional literature does not call him a saint, referring to him as a patriarch instead, and he has no churches named for him. A 15th century stained glass window in the parish church at St Neot in Cornwall, supposedly depicts Brychan, seated and crowned, holding in his arms eleven children. This, however, has been described by a standard modern guide as "God the Father with souls in his lap".