The River Fergus (Irish: An Forghas) is a river within the Shannon River Basin which flows in County Clare, Ireland. It rises in Loughnagowan which lies a few kilometers west of Corofin and flows into the Shannon Estuary. It is noted for its trout and salmon fishing. Trummer is an island in the river. There are six bridges crossing it in Ennis alone.
The River Fergus has an average discharge of 25.7m3/s
Settlements along the river include Corofin, Ennis, Clarecastle and Newmarket-on-Fergus.
- ^  -River Fergus, The Fishing House
- ^ Long-term effects of hydropower installations and associated river regulation on River Shannon eel populations: mitigation and management 
Coordinates: 52°42′N 9°00′W / 52.700°N 9.000°W