Berthier-sur-Mer (French pronunciation: [bɛʁtje syʁ mɛʁ], lit. 'Berthier on Sea') 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.

• List of municipalities in Quebec