Lake Ontario, bounded on the north by Ontario and on the south by Ontario's Niagara Peninsula and by New York State, is one of the five Great Lakes of North America.

Lake Ontario is the eastern-most and smallest in surface area (19,009 square kilometers) of the Great Lakes. It exceeds only Lake Erie in volume (1639 km³). Its primary inlet is Niagara Falls (from Lake Erie) and primary outlet is the Saint Lawrence River.

A portion of the Great Lakes Waterway passes through the lake, which is accessible from upstream by the Welland Canal and from downstream by the Saint Lawrence Seaway. The Trent-Severn Waterway for pleasure boats connects Lake Ontario to Georgian Bay of Lake Huron through Lake Simcoe. The Rideau Waterway, also for pleasure boats, connects Lake Ontario at Kingston to the Ottawa River at Ottawa.

Other notable geographic features are Hamilton Harbour, the Bay of Quinte, and the Toronto Islands.

The American shore of the lake is largely rural, with the exception of Rochester, New York. A large conurbation called the Golden Horseshoe (including Toronto and Hamilton, Ontario) is to be found on the Canadian side.

Great Lakes
Lake Superior	Lake Michigan	Lake Huron	Lake Erie	Lake Ontario