Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north and southwest by the Canadian province of Ontario, and on the south and east by the American state of New York. Ontario, Canada's most populous province, was named for the lake. In the Wyandot (Huron) language, ontarío means "Lake of Shining Waters". It is the last in the Great Lakes chain and serves as the outlet to the Atlantic Ocean via the St. Lawrence River. Lake Ontario is also the only one of the five Great Lakes not to share a shoreline with the American state of Michigan.

Lake Ontario is the easternmost of the Great Lakes and the smallest in surface area (7,340 sq mi, 18,960 km2),although it exceeds Lake Erie in volume (393 cu mi, 1,639 km3). It is the 14th largest lake in the world.