Cantref in central Wales, containing the source and upper reaches of the Severn, and part of the upper reaches of the Wye.
It lay on the southern borders of the principality of Powys, but, as a buffer territory, it managed to keep its own dynasty of princes until nearly the end of the 12th century. Not only Powys, but at times the Welsh princes of Deheubarth and of Gwynedd, and the English or Norman holders of Hereford interfered in its affairs.
Eventually the new Prince of South Powys, Gwenwynwyn, annexed Arwystli in 1197, but he was soon in trouble with King John, and was defeated and dethroned in 1208. He was restored in 1210, but was under the dominance of England, then of Gwynedd before his death in 1216. Arwystli was controlled by either South Powys or Gwynedd during the 13th century. The Prince of Gwynedd’s awareness of his rights there was enhanced by the fact that Arwystli was a detached part of the Bishopric of Bangor, which was the diocese for Gwynedd.
A dispute over Arwystli in the 1270s helped bring about the destruction of independent Wales, for King Edward I insisted that the matter be resolved in the English courts, according to English law – acceptable to Powys but not to Gwynedd. After the destruction of Gwynedd’s independence, Arwystli remained in what became the marcher lordship of (South) Powis until 1536, when it became the south of the county of Montgomeryshire.