| Lemnia | |
| Pushpin Map: | Romania |
| Subdivision Type1: | Country |
| Subdivision Name1: | Romania |
| Subdivision Type2: | Counties |
| Subdivision Name2: | Covasna County |
| Subdivision Type3: | Villages |
| Subdivision Name3: | Lemnia |
| Length: | 13km (08miles) |
| Mouth: | Râul Negru |
| Mouth Location: | Lemnia |
| Mouth Coordinates: | 46.0378°N 26.2589°W |
| Basin Size: | 35km2 |
| Tributaries Left: | Tisa |
The Lemnia is a right tributary of the Râul Negru in Romania. It flows into the Râul Negru near the village Lemnia. Its length is 13km (08miles) and its basin size is 35km2.