| Berthier-sur-Mer | |
| Settlement Type: | Municipality |
| Motto: | Sur terre et sur mer, Berthier toujours plus fier |
| Pushpin Map: | Canada Southern Quebec |
| Pushpin Map Caption: | Location in province of Quebec. |
| Coordinates: | 46.9167°N -114°W |
| Subdivision Type: | Country |
| Subdivision Type1: | Province |
| Subdivision Type2: | Region |
| Subdivision Name2: | Chaudière-Appalaches |
| Subdivision Type3: | RCM |
| Subdivision Name3: | Montmagny |
| Established Title1: | Constituted |
| Established Date1: | July 1, 1855 |
| Leader Title: | Mayor |
| Leader Name: | Richard Gallibois |
| Leader Title1: | Federal riding |
| Leader Name1: | Montmagny—L'Islet— Kamouraska—Rivière- du-Loup |
| Leader Title2: | Prov. riding |
| Leader Name2: | Côte-du-Sud |
| Area Total Km2: | 26.80 |
| Area Land Km2: | 26.85 |
| Population Total: | 1744 |
| Population As Of: | 2021 |
| Population Density Km2: | 65 |
| Population Blank1 Title: | Pop 2016-2021 |
| Population Blank1: | 12.2% |
| Population Blank2 Title: | Dwellings |
| Population Blank2: | 939 |
| Timezone: | EST |
| Utc Offset: | −5 |
| Timezone Dst: | EDT |
| Utc Offset Dst: | −4 |
| Postal Code Type: | Postal code(s) |
| Postal Code: | G0R 1E0 |
| Area Codes: | 418 and 581 |
| Blank Name: | Highways |
| Blank Info: | |
Berthier-sur-Mer is a town in the Montmagny Regional County Municipality within the Chaudière-Appalaches region of Quebec, Canada. It is located on the south shore of the Saint Lawrence River, east of Quebec City on Route 132.
Berthier-sur-Mer was named after Alexandre Berthier, a captain in the Carignan-Salières Regiment, who was given title to this area in 1672, then known as the seigneury of Berthier-en-bas.
During the 19th century, Irish immigrants were quarantined at nearby Grosse Isle, now a National Historic site.
A strip of rather interesting underlying sedimentary bedrock maintains a relatively straight shoreline through which a natural harbour, quite round in shape, has been eroded. The bedrock is set at about a 60 to 80° angle and is composed of hundreds of very thin layers of red shale interspersed every meter or so with 5 to 10 cm layers of what appears to be limestone. It is quite dramatic in places where it hasn't been worn down by human activities.