Lake Ontario is located between the United States and Canada.

It forms part of the border between the two countries, with the U.S. state of New York on the south shore and the Canadian province of Ontario on the north shore. Lake Ontario is the easternmost and smallest of the Great Lakes and serves as the outlet of the entire Great Lakes system, with water flowing from it into the St. Lawrence River. Its location and connection to both nations make it an important waterway for trade, transportation, and ecological management.