| Lake Õisu | |
| Pushpin Map: | Estonia |
| Location: | Ereste, Mulgi Parish. Viljandi County, Estonia |
| Basin Countries: | Estonia |
| Area: | 193.4ha |
| Depth: | 2.8m (09.2feet) |
| Max-Depth: | 4.3m (14.1feet) |
| Elevation: | 45.5m (149.3feet) |
Lake Õisu is a lake in Mulgi Parish, Viljandi County, in southern Estonia. Administratively it's located in Ereste village, just west of Õisu small borough. The lake and its surroundings are covered by the Õisu Landscape Conservation Area.