| Rinn Lough | |
| Pushpin Map: | Island of Ireland |
| Pushpin Map Alt: | Rinn Lough location in Ireland |
| Pushpin Label Position: | none |
| Pushpin Map Caption: | Location in Ireland |
| Location: | County Leitrim |
| Inflow: | Cloone River |
| Outflow: | River Rinn |
| Catchment: | 178.06km2 |
| Basin Countries: | Ireland |
| Length: | 4km (02miles) |
| Width: | 0.7km (00.4miles) |
| Area: | 1.65km2 |
| Elevation: | 39m (128feet) |
| Islands: | 6 |
| Cities: | Mohill |
Rinn Lough (also), also known as Lough Rynn, is a freshwater lake in the northwest of Ireland. It is located in south County Leitrim.
Rinn Lough measures about 4km (02miles) long and 1km (01miles) wide. It is located about 4km (02miles) south of Mohill. Lough Rynn Castle, a medieval castle now a hotel, occupies an estate on the lake's northeastern shore.
Two smaller neighbouring lakes drain into Rinn Lough: Clooncoc Lough and Lough Errew. Rinn Lough drains south into the Rinn River flowing into Lough Forbes. Part of the river forms the Rinn River Natural Heritage Area.
The water quality was reported to be satisfactory maintaining a mesotrophic rating. but given a "" due to pollution. Zebra mussel infestation is present. The ecology of Rinn Lough, and Irish waterways, remains threatened by curly waterweed, zebra mussel, and freshwater clam invasive species.