Segontium (Old Welsh: Cair Segeint) is a Roman fort on the outskirts of Caernarfon in Gwynedd, North Wales.

Unlike the medieval Caernarfon Castle that was built alongside the Seiont estuary more than a thousand years later, Segontium was situated on higher ground to the east giving a good view of the Menai Straits.

[3] An inscription on an aqueduct from the time of the Emperor Septimius Severus indicates that, by the 3rd century, Segontium was garrisoned by 500 men from the Cohors I Sunicorum, which would have originally been levied among the Sunici of Gallia Belgica.

At this time Segontium's main role was the defence of the north Wales coast against Irish raiders and pirates.

[6] Constantius Chlorus actually died at York; the Welsh monument might be for Constantine[5] who was the son of Saint Elen, the supposed patron of the Sarn Helen.

The remains of a civilian settlement together with a Roman temple of Mithras, the Caernarfon Mithraeum, and a cemetery have been also identified around the fort.