The Ontario Peninsula is the southernmost part of the province of Ontario and of Canada as a whole. It is bounded by Lake Huron to the west, Lake Ontario to the east, and Lake Erie to the south. At its tip, the peninsula is separated from Michigan by the Detroit River, the St. Clair River, and Lake St. Clair. A secondary peninsula projects toward New York on its eastern side and ends at the Niagara River.

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The corner of the peninsula that lies on Lake Ontario is known as the Golden Horseshoe and forms Canada's largest population centre. Other large cities include London and Windsor.

Like other parts of southern Canada, the Ontario Peninsula enjoys warm or hot summers with normal thunderstorm activity, including severe thunderstorms that can have hail, damaging winds, and even tornadoes during peak season. It has cold winters, and snowfall can be abundant, particularly in the affected snowbelt locations. However, there are many winter thaw periods that break the entrenched cold. The Ontario Peninsula has a humid continental climate; specifically, most of it falls into the Köppen climate classification Dfb except for Essex County, Chatham-Kent, and parts of the Greater Toronto and Hamilton Area and the Niagara Peninsula, which are within the Dfa zone. However, the entire peninsula is near the Dfa/Dfb borderline.

Anishinaabe tribes, particularly the Mississaugas, are indigenous to the Ontario Peninsula.^[1][2] Southern Ontario was colonized by France in the 1600s, but the British gained control of Ontario after the Seven Years' War.

The Ontario Peninsula experienced much of the fighting during the War of 1812,^[3] including the Americans invading it and burning York (now called Toronto). After the war, population and trade boomed, and the Welland Canal was built.

When the British divided Canada into separate provinces, Toronto became the capital of Ontario and eventually became the main economic centre of Canada.

By each definition, Toronto or its corresponding entity is the most populous not only in the Ontario Peninsula but also in Canada as a whole.