 Aerial view of Caer Gai | |
 Location of Caer Gai within the Roman Province of Britannia | |
| Location | Penllyn, Gwynedd |
|---|---|
| Region | Wales |
| Coordinates | 52°52′6.312″N 3°40′10.74″W / 52.86842000°N 3.6696500°W |
| Altitude | 200 m (656 ft) |
| Type | fort |
| Area | 4 acres (1.6 ha) |
| History | |
| Builder | Roman Army |
| Material | First fort: earth and wood Second fort: stone |
| Founded | 1st - 2nd centuries CE |
| Abandoned | 5th century CE |
| Periods | Classical antiquity |
| Cultures | Roman Britain |
| Associated with | Auxilia (Nervii) |
| Site notes | |
| Condition | earthworks |
| Management | CADW |
| Designation | ME018 |
Caer Gai (also Caer-gai) is a Grade II listed Roman fort in the district of Penllyn, Gwynedd, Wales, UK. It is located about 1 mile or 1.6 km north of the village of Llanuwchllyn, and the same distance west of Lake Bala.