Saint Tewdrig was a 7th century King of Glywysing (Glamorgan) and Gwent in South Wales who was martyred fighting the Anglo-Saxons.

Tewdrig was the son of the previous king, either Llywarch or Teithfall, and a supposed benefactor of the church of Llandaff. He resigned the government to his son, Meurig, in order to devote himself to religion and contemplation at Tintern in Monmouthshire. However, when the Saxons, under King Ceolwulf, crossed the Severn and pressed hard upon Meurig, Tewdrig left his solitude and gained a victory at the head of his old troops, but was killed in the process. A church was erected over his grave and called Marthyr Tewdrig. This is now Mathern, at the junction of the Rivers Wye and Severn. He was the grandfather of Athrwys ap Meurig.