Ontario, Lake, the smallest and most easterly of the Great Lakes of North America. It lies north and east of Niagara Falls, between New York and the Canadian province of Ontario. Greatest length (east-west) is 193 miles (311 km), and greatest width is 53 miles (85 km). It has an area of 7,340 square miles (19,000 km2), of which 3,460 square miles (9,000 km2) belong to the United States. The surface is 320 feet (98 m) below that of Lake Erie, and 250 feet (76 m) above sea level. The greatest depth is 800 feet (244 m).

Lake Ontario is subject to swells and storms that prevent its freezing over in winter. Ice is broken in the ports in midwinter, and shipping is possible on the lake throughout the year.

The chief Canadian ports are Hamilton, Toronto, and Kingston. On the New York side Oswego and Rochester are important ports. Lake Ontario receives the waters of the western Great Lakes through the Niagara River, and empties into the St. Lawrence River at the northeastern end of the lake. The chief rivers emptying into Lake Ontario are the Black, Genesee, and Oswego in New York, and the Trent in Ontario.

The first Europeans to visit the lake were Étienne Brulé and Samuel de Champlain in 1615.