The St. Lawrence Seaway is the common name for a system of canals that permits ocean-going vessels to travel from the Atlantic Ocean to the Great Lakes, as far as Lake Superior. Legally it extends from Montreal to Lake Erie, including the Welland Canal and the Great Lakes Waterway. The seaway is named after the Saint Lawrence River, which it follows from Lake Ontario to the Atlantic Ocean.

Beginning in the Saint Lawrence River, the seaway leaves Quebec through Lac Saint-François, passing through the Akwesasne Mohawk First Nation. At Cornwall and Massena, the river and seaway form an international waterway, with Ontario on the northern shore and New York State on the southern shore. Cornwall and Massena are joined by an international bridge over the St. Lawrence, the Seaway International Bridge, named for the seaway.

Between Cornwall and Massena, the seaway diverges south of the river, passing through New York's Wiley-Dondero Canal, containing the Snell and Eisenhower locks. This route bypasses the Moses-Saunders hydroelectric dam before passing through the short Iroquois Lock on the Canadian side of the river, permitting vessels to bypass the Iroquois water level control structure. West of the Iroquois Lock, the seaway follows the St. Lawrence River through the Thousand Islands and into Lake Ontario. Altogether there are seven locks in the Montreal–Lake Ontario section (five Canadian, two American).