Lake Ontario, Great lake between Ontario, Canada and New York, United States.

Lake Ontario is a body of water in Ontario, Canada, and New York, United States, spanning nearly 19,000 square kilometers with an average depth of 86 meters (282 feet). The deepest point reaches 244 meters (800 feet), and it forms an important part of the Great Lakes system in North America.

The basin was carved from Silurian-age rock by the Wisconsin ice sheet along the pre-glacial Ontarian River valley. While the retreating ice still dammed the St. Lawrence outlet, the higher Lake Iroquois stage drained south through present-day Syracuse to the Mohawk and Hudson rivers. The modern level and outlet formed about 11,000 years ago; Samuel de Champlain named the lake Lac S. Louis in 1632, and the name Ontario was documented by 1641.

The name Ontario probably comes from the Iroquoian Kaniatarí:io or Oniatarí:io, "lake of shining waters" or "beautiful lake", documented since 1641. Samuel de Champlain had named it Lac S. Louis in 1632, and French colonial Canada also used Frontenac and St. Louis. In August 2026, amid a trade war with Canada, US President Donald Trump ordered federal agencies to call it Lake America; Canadian and many US state officials rejected the name, but Apple and Google mapping services adopted it for US users.

The lake is oriented west to east, with a constant surface current flowing from the Niagara River toward the St. Lawrence River at about 8 miles per day, faster along the south shore. The Welland Canal links it to Lake Erie, and the Trent Canal connects it to Georgian Bay at Trenton. The central lake rarely freezes, though coastal waters freeze from mid-December to mid-April.

The lake's basin was carved by the Wisconsin ice sheet along the pre-glacial Ontarian River valley. When the ice retreated, the higher Lake Iroquois stage drained through present-day Syracuse, New York, to the Mohawk and Hudson rivers; its dry beaches and wave-cut hills lie 16 to 40 kilometres from today's shoreline.