The outlet of Lake Ontario is the St. Lawrence River.

The St. Lawrence River flows eastward from Lake Ontario, connecting it to the Atlantic Ocean. It serves as the primary outlet for the Great Lakes system, allowing water from Lake Ontario to flow out and eventually reach the sea. The river forms part of the border between the United States and Canada and is a major waterway for shipping, transportation, and commerce, linking the Great Lakes to international markets. The St. Lawrence River also plays a crucial role in the region's ecosystem and economy.