Monterrein

{{Infobox French commune

|name = Monterrein

|native name = Mousterrin

|image = Église St-Malo de Monterrein (Morbihan).JPG

|caption = The church of Saint-Malo, in Monterrein

|image coat of arms = Blason ville fr Monterrein (Morbihan).svg

|coordinates = {{coord|47.8811|-2.3566|format=dms|display=inline,title}}

|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 = [https://www.insee.fr/fr/statistiques/fichier/4989724/ensemble.pdf Téléchargement du fichier d'ensemble des populations légales en 2018], INSEE

}}

Monterrein ({{IPA|fr|mɔ̃tɛʁɛ̃}}; {{Langx|br|Mousterrin}}) is a former commune in the Morbihan department of Brittany in north-western France.{{Cassini-Ehess|23366|Monterrein}} On 1 January 2019, it was merged into the commune Ploërmel.[https://www.legifrance.gouv.fr/eli/arrete/2018/12/21/TERB1905216A/jo/texte Arrêté préfectoral] 21 December 2018 {{in lang|fr}} Inhabitants of Monterrein are called in French Monterrinois.