Ystradyfodwg or Ystrad Dyfodwg (Vale of Tyfodwg) was an ancient upland parish in Glamorgan, Wales. It is believed to have been named after Dyfodwg (or Tyfodwg) a 6th-century saint or chieftain. The parish included most of the area which would later be known as Rhondda named for the parish rivers, Rhondda Fawr and Rhondda Fach.