Pen Rhionydd is named as the location of King Arthur's northern court in a Welsh triad found in Peniarth MS 54, containing pre-Galfridian traditions:

Arthur as Chief Prince in Pen Rhionydd in the North, and Gerthmwl Wledig as Chief Elder, and Cyndeyrn Garthwys as Chief Bishop.^[1]

There are no other known references to this location in Arthurian literature. The same triad goes on to say Arthur's other courts were at Celliwig and Mynyw.

A possible location, supported by Rachel Bromwich, the latest editor of the Welsh Triads, is a location somewhere near the Rhins of Galloway and Stranraer.^[2] This would match the importance of St Mungo in that area. Both these places would have been in Rheged^{[according to whom?]}.