Cynog son of Brychan (Welsh: Cynog ap Brychan; born c. 434), better known as Saint Cynog (Old Welsh: Kennauc), was an early Welsh saint and martyr. His shrine is at Merthyr Cynog in Wales and his feast day is observed on 7 or 9 October. Cynog was reputedly a son of Brychan, a powerful Welsh prince of the British Dark Ages. Cynog's mother was reputedly , daughter of , a prince of Powys, whom Brychan seduced while a hostage at the court of her father. He is said to have been murdered on the mountain called the Van (Bannau Brycheiniog). His relics are housed at Merthyr Cynog.