Cefn Carnedd is an Iron Age hillfort, about south-west of Caersws, in Powys, Wales. It is a scheduled monument.
It is on a hill overlooking the River Severn to the east, and Afon Cerist and Afon Trannon to the north; there is a river confluence to the north-east, between the fort and Caersws.
The fort is about by orientated north-east to south-west, aligned with the shape of the hill; it encloses an area of about . There are triple banks and ditches on the north-west side, and there are entrances in the north-east and south-west.
In an early phase, there was a smaller fort of at the south-west end, traces of the north-east rampart of this being visible in aerial photographs. There is a straight bank and ditch across the interior, which was built at a late stage.
It is thought that the fort may have been a stronghold of the Ordovices, and it is one of the locations suggested as the site of Caratacus' last battle in AD 51, when he was defeated by the Romans.