| Montmartin-sur-Mer | |
| Commune Status: | Commune |
| Arrondissement: | Coutances |
| Canton: | Quettreville-sur-Sienne |
| Insee: | 50349 |
| Postal Code: | 50590 |
| Mayor: | Bruno Quesnel[1] |
| Term: | 2020 - 2026 |
| Intercommunality: | Coutances Mer et Bocage |
| Coordinates: | 48.9894°N -1.5233°W |
| Elevation M: | 50 |
| Elevation Min M: | 2 |
| Elevation Max M: | 60 |
| Area Km2: | 9.81 |
Montmartin-sur-Mer is a commune in the Manche department in Normandy in north-western France.