Cyfeiliog was a medieval commote in the cantref of Cynan of the Kingdom of Powys. Cynan also contained the commote of Mawddwy. Other sources refer to Cyfeiliog as a cantref in its own right, possibly as a result of Cynan being renamed for the largest commote within it. It bordered the cantrefi of Penllyn in the north, Caereinion in the east and Arwystli in the south-east. Its border in the north-east was with the cantref of Meirionydd in the Kingdom of Gwynedd, and its south-east border was with the cantref of Penweddig in the Kingdom of Ceredigion.