The St. Lawrence Seaway (French: la Voie Maritime du Saint-Laurent) is a system of locks, canals, and channels in Canada and the United States that permits oceangoing vessels to travel from the Atlantic Ocean to the Great Lakes of North America, as far inland as Duluth, Minnesota, at the western end of Lake Superior. The seaway is named for the St. Lawrence River, which flows from Lake Ontario to the Atlantic Ocean. Legally, the seaway extends from Montreal, Quebec, to Lake Erie, and includes the Welland Canal. Ships from the Atlantic Ocean are able to reach ports in all five of the Great Lakes.

The St. Lawrence River portion of the seaway is not a continuous canal; rather, it consists of several stretches of navigable channels within the river, a number of locks, and canals along the banks of the St. Lawrence River to bypass several rapids and dams. A number of the locks are managed by the St. Lawrence Seaway Management Corporation in Canada, and others in the United States by the St. Lawrence Seaway Development Corporation; the two bodies together advertise the seaway as part of "Highway H2O". The section of the river from Montreal to the Atlantic is under Canadian jurisdiction, regulated by the offices of Transport Canada in the Port of Quebec.

Massena is a town in St. Lawrence County, New York, United States. Massena is along the county's northern border, just south of the St. Lawrence River and the Three Nations Crossing of the Canada–United States border. The population was 12,883 at the 2010 census. The town of Massena contains a village also named Massena.