The Gulf of Saint Lawrence is bounded on the north by the Labrador Peninsula and Quebec, to the east by Saint-Pierre and Newfoundland, to the south by the Nova Scotia peninsula and Cape Breton Island, and to the west by the Gaspe Peninsula, New Brunswick, and Quebec. As for significant islands the Gulf of Saint Lawrence contains Anticosti Island, PEI, Îles-de-la-Madeleine, Cape Breton Island, Saint Pierre Island, and Miquelon-Langlade.
Half of the ten provinces of Canada adjoin the Gulf: New Brunswick, Nova Scotia, Prince Edward Island, Newfoundland and Labrador, and Quebec.
Besides the Saint Lawrence River itself, significant streams emptying into the Gulf of Saint Lawrence include the Miramichi River, Natashquan River, Romaine River, Restigouche River, Margaree River, and Humber River.
Branches of the Gulf include the Chaleur Bay, Fortune Bay, Miramichi Bay, St. George's Bay, Bay St. George, Bay of Islands, and Northumberland Strait.
The gulf flows into the Atlantic Ocean through the following outlets:
The limits of the Gulf of Saint Lawrence vary between sources.
The International Hydrographic Organization defines the limits of the Gulf of Saint Lawrence as follows:
Fisheries and Oceans Canada places the western limit at Pointe-des-Monts.
St. Paul Island, Nova Scotia, off the northeastern tip of Cape Breton Island, is referred to as the "Graveyard of the Gulf" because of its many shipwrecks. Access to this island is controlled by the Canadian Coast Guard.
Bonaventure Island on the eastern tip of the Gaspe Peninsula, Île Brion and Rochers-aux-Oiseaux (Bird Rock) northeast of the Magdalen Islands are important migratory bird sanctuaries administered by the Canadian Wildlife Service.
The Federal Government of Canada has national parks along the Gulf of Saint Lawrence at Forillon National Park on the eastern tip of the Gaspe Peninsula, Prince Edward Island National Park on the northern shore of the island, Kouchibouguac National Park on the northeastern coast of New Brunswick, Cape Breton Highlands National Park on the northern tip of Cape Breton Island, Gros Morne National Park on the west coast of Newfoundland, and a National Park Reserve in the Mingan Archipelago on the Côte-Nord of Quebec.
The five provinces bordering the Gulf of Saint Lawrence also have several provincial parks apiece, some of which preserve coastal features.
The Laurentian Channel is a feature of the floor of the Gulf that was formed during previous ice ages, when the Continental Shelf was eroded by the Saint Lawrence River during the periods when the sea level plunged. The Laurentian Channel is about 290 m (950 ft) deep and about 1,250 km (780 mi) long from the Continental Shelf to the mouth of the Saint Lawrence River. Deep waters with temperatures between 2 and 6.5 °C (36 and 44 °F) enter the Gulf at the continental slope and are slowly advected up the channel by estuariane circulation. Over the 20th century, the bottom waters of the end of the channel (i.e. in the Saint Lawrence estuary) have become hypoxic.
The gulf has provided a historically important marine fishery for various First Nations that have lived on its shores for millennia and used its waters for transportation.
The first documented voyage by a European in its waters was by the French explorer Jacques Cartier in the year 1534. Cartier named the shores of the Saint Lawrence River "The Country of Canadas", after an indigenous word meaning "village" or "settlement", thus naming the world's second largest country.
At just about the same period, Basques came to frequent the area for whale-hunting and trade with the First Nations people of the modern Canadian Atlantic and Quebec provinces. They left vestiges of their presence in many locations of the area—docks, furnaces, graveyards, etc.