St Lawrence (French, St-Laurent), river in eastern North America, about 1,200 km (745 mi) long, the chief outlet of the Great Lakes. The St Lawrence River system, including the Great Lakes, is about 4,000 km (2,486 mi) long; its source is the St Louis River, in Minnesota. The total catchment area of the river (including that of all the Great Lakes) is 1,424,000 sq km (550,000 sq mi), and the average discharge at the river's mouth is 14 million litres (3.1 million gallons) per second.

The St Lawrence proper rises in north-eastern Lake Ontario and flows north-east to the Gulf of St Lawrence, at Anticosti Island. In its upper course, which includes the scenic Thousand Islands, the river forms the boundary between Ontario and New York state. It then widens into Lake St Francis, which serves for a short distance as the border between Ontario and Quebec before flowing wholly in Quebec for the remainder of its course. At Montreal the river expands to create Lake St Louis, and below Sorel it forms the much larger Lac Saint-Pierre (Lake St Peter). Below Quebec City the St Lawrence broadens from a width of about 3 km (2 mi) to one of 145 km (90 mi) at its mouth, where it discharges great quantities of water. Ocean tides extend up the river for about 800 km (500 mi), to the city of Trois-Rivières. The main tributaries of the St Lawrence are the Ottawa, Saint-Maurice, and Saguenay rivers from the north, and the Richelieu, St-François (St Francis), and Chaudière rivers from the south.

The St Lawrence is navigable by large ocean-going vessels as far upstream as Montreal, and somewhat smaller ships can continue to Lake Ontario via the St Lawrence Seaway. Ice accumulations prevent navigation on the upper river from December to April. The St Lawrence Valley's farms produce dairy goods, fruit, potatoes, and grain.

The first European to visit the St Lawrence River was the French explorer Jacques Cartier, in 1534. He sailed as far as the site of present-day Montreal, having named the river after the saint on whose feast day (August 10) they had entered its mouth. The first permanent European settlement on its banks was established in 1608 at present-day Quebec City by Samuel de Champlain.