Saint Tydfil (standard Welsh Tudful; martyred ca. 480) is the legendary dedicatee of Merthyr Tydfil (Welsh: Merthyr Tudful, "Martyr Tydfil"), a town in Glamorgan, south Wales.
Tydfil gave her name to Merthyr Tydfil (Merthyr meaning martyr in the Welsh language).
Tydfil was the daughter of King Brychan by his fourth wife. Most of Brychan's children were well educated, girls and boys, at a school in Gwenddwr on the Wye and went on to live deeply religious lives. Her martyrdom took place during a pitched battle between her family and a band of marauding Picts during the fifth century AD. Although much of what is known about her comes from monks writing long after she was supposed to have lived, evidence shows that she did exist and that she did meet with a violent end.
"It was said that here [at Merthyr Tudful] were the remains of a female saint called Tudful who was martyred by invading ‘Irish and Picts’ in the fifth century. The name Tudful though seems more likely to have been a man’s name." But local legends say that Tydfil (a dialectal variant of Tudful) was one of the many holy daughters of King Brychan of Brycheiniog, killed by pagans (probably Welsh, although they were later said to have been Saxons) at Merthyr Tudful around 480.