| Monterrein | |
| Native Name: | Mousterrin |
| Image Coat Of Arms: | Blason ville fr Monterrein (Morbihan).svg |
| Coordinates: | 47.8811°N -2.3566°W |
| Insee: | 56138 |
| Postal Code: | 56800 |
| Arrondissement: | Pontivy |
| Canton: | Ploërmel |
| Commune: | Ploërmel |
| Elevation Min M: | 59 |
| Elevation Max M: | 126 |
| Area Km2: | 7.01 |
| Population: | 396 |
| Population Date: | 2018 |
| Population Footnotes: | [1] |
Monterrein (pronounced as /fr/;) is a former commune in the Morbihan department of Brittany in north-western France. On 1 January 2019, it was merged into the commune Ploërmel.[2] Inhabitants of Monterrein are called in French Monterrinois.