The St. Marys River (or St. Mary's River) may refer to:

• St. Marys River in Maryland in the United States.
• St. Marys River separating Georgia and Florida in the United States.
• St. Marys River in Indiana in the United States.
• St. Marys River is a river connecting Lake Superior and Lake Huron, flowing between Michigan and Ontario.

See also St. Mary River