Saint Dyfan is a highly obscure figure who was presumably the namesake of Merthyr Dyfan ("martyrium of Dyfan") and therefore an early Christiansaint and martyr in southeastern Wales in Roman or Sub-Roman Britain. He is sometimes styled the protomartyr of Wales. The erection of his martyrium was credited to the 6th-century StTeilo. In the 19th century, Edward Williams conflated him with StDeruvian, a figure in the legendary accounts of the baptism of KingLucius of Britain. The discovery of Williams's alterations and forgeries have since discredited this connection. Partially based on this connection, however, the church of Merthyr Dyfan dates his martyrdom to c.180.
The parish church of Merthyr Dyfan is now dedicated jointly to SSDyfan and Teilo. As late as 2010, it continued to promote itself as the oldest Christian settlement in Wales on the basis of Dyfan's supposed connection to the KingLucius legends.