Segontium is a Roman fort for a Roman auxiliary force, located on the outskirts of Caernarfon in Gwynedd, north Wales.
It probably takes its name from the nearby River Seiont, and may be related to the Segontiaci, a British tribe mentioned by Julius Caesar. The fort was founded by Agricola in 77 or 78 AD after he had conquered the Ordovices. It was the main Roman fort in the north of Roman Wales and was designed to hold about a thousand auxiliary infantry. It was connected by a Roman road to the Roman legionary base at Chester, Deva Victrix. Unlike the more recent Caernarfon Castle alongside the Seiont estuary, Segontium is located on higher ground giving a good view of the Menai Straits.
The original timber defences were rebuilt in stone in the first half of the second century AD. An inscription on an aqueduct from the time of the Emperor Septimius Severus indicates that at that time it was garrisoned by Cohors I Sunicorum, which would have originally been levied among the Sunici of Gallia Belgica.
The site is now cut through by the A4085 road to Beddgelert, but the remains of most of the buildings are preserved. There is a visitor centre and a small museum exhibiting finds made in and around the fort. Outside the fort, the remains of a civilian settlement have been found, together with a Roman temple of Mithras, the Caernarfon Mithraeum and a cemetery.
Segontium is implicit in the name of the surrounding town, because "caer" means fort. The name of the town of Caernarfon is the corrupted form of "Caer yn ar-Fon", which means "Fort in (the land) opposite Mon".