Ontario, Lake, lake, central North America, bordered on the north by the Canadian province of Ontario and on the south by the state of New York. About 310 km (about 190 mi) long and up to about 85 km (about 53 mi) wide, the lake covers an area of 19,011 sq km (7340 sq mi), slightly more than half of which is in Canada. It is the smallest and most easterly of the five Great Lakes.

Lake Ontario is fed by several rivers, including the Niagara River, which flows north from Lake Erie; the Genesee, Oswego, and Black rivers, which enter from New York; and the Trent and Humber rivers, which enter from Ontario. The lake discharges into the Saint Lawrence River to the northeast. A major shipping route, it is an important component of the Saint Lawrence Seaway, and is linked with Lake Erie by the Welland Canal and with Lake Huron by the Trent Canal system. In addition, Lake Ontario is connected with the Ottawa River by the Rideau Canal and with the Hudson River by the New York State Barge Canal system. Major ports on the lake include Oswego and Rochester, in New York, and Hamilton, Kingston, and Toronto, in Ontario. During the winter months the area near the shore freezes over, and many harbors on the lake are icebound from about mid-December to mid-April.