Edeirnion or Edeyrnion is an area of the county of Denbighshire and an ancient commote of medieval Wales in the cantref of Penllyn. According to tradition, it was named after its eponymous founder Edern or Edeyrn. It was included as a Welsh territory of Shropshire in the Domesday Book.
Edeirnion was nominally a part of the Kingdom of Powys but was often subject to border intrusions by the neighbouring Kingdom of Gwynedd. It was the patrimony of prince Owain Brogyntyn. These rumbling border disputes caused a great deal of friction between the two realms. Edeirnion was occupied and annexed by Gwynedd in the reign of Llywelyn the Great but briefly returned to Powys following a treaty forced on Gwynedd by England after Llywelyn's death in 1240. The territory was again occupied by Gwynedd after 1267 before being returned again to Powys. This continuing dispute and the appeal by Llywelyn ap Gruffudd to Edward I of England to see the resolution of this dispute settled by Welsh Law was one of the reasons the principalities in the north of Wales were unable to unite in opposition to English hegemony and was a contributing factor to the final war between the Principality of Wales and England, which ultimately saw the end of Welsh independence.
Edeirnion still exists as a bro, or region, in Denbighshire, located around Corwen and near the Berwyn Range.