The Maen Madoc or Maen Madog stone is a menhir which lies adjacent to the Roman road Sarn Helen that runs across the Brecon Beacons in what was a key area of Roman Wales, about one mile (2 km) north of Ystradfellte.

It is thought to have marked a late Roman - post-Roman Christian burial – the stone is inscribed on one side, the Latin inscription reading DERVAC(IVS) FILIVS IVSTI (H)IC IACIT – "Of Dervacus, Son of Justus.

Excavations undertaken at the time showed that the stone had previously been sited at the edge of the Roman road, with no grave associated.

[1] Maen Madoc can be found at SN918157 on the Brecon Beacons West and Central map.

It has been recorded by the Royal Commission on Ancient and Historical Monuments in Wales (NPRN 84540),[2] Early Christian Monuments in Wales ECM 8, and Clwyd-Powys Archaeological Trust Historic Environment Record Primary Record Number 825.