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The Ontario Peninsula is the southernmost part of the province of Ontario and of Canada as a whole. It is bounded by Lake Huron on the west, Lake Ontario on the east, and Lake Erie on the south. At its tip, it is separated from Michigan by the Detroit and St. Clair rivers, as well as Lake St. Clair. The peninsula also includes the Bruce and Niagara peninsulas, one projecting into Lake Huron and the other projecting towards New York, from which it is separated by the Niagara River.

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 often passing 30 °C (86 °F) and rarely 40 °C (104 °F) during extreme heatwaves. During the summer, the peninsula has 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.

Several countries covered the territories of the Ontario Peninsula before European colonization, most notably the United Nations of Ojibwe, Ottawa, and Potawatomi, also called the Council of Three Fires, Huronia (or Wendake), and the Neutral Confederacy. In the 1600s, Southern Ontario would be colonized by France, with particular efforts to build alliances with each of the nations in the region they called the Pays d'en Haut, including setting up a mission in the Wendat (Huron) capital of Ossossané in what is now Simcoe County.^[1] These alliances were crucial as the French were perpetual enemies of the Haudenosaunee, with their country, Iroquoia, situated just east of Lake Ontario. Although every Indigenous nation was hit hard by outbreaks of disease, some of the biggest changes came during the Beaver Wars, which only came to an end in 1701 at the signing of the Great Peace of Montreal. In the aftermath, Wendake, the Neutral Confederacy, and Petun Country were destroyed, and demographic shifts saw the Ojibweg, notably those who were originally from around the Mississagi River, move onto the lands immediately north of Lake Ontario; these Anishinaabe would adopt the name "Mississauga."^[2][3] It is from them that the British bought land as part of the 1784 Between the Lakes Treaty to grant to the Kanien'kehá:ka as part of the Haldimand Proclamation, after the Seven Years' War, establishing the Six Nations of the Grand River.

Later, the Ontario Peninsula would experience much of the fighting during the War of 1812,^[4] 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 the Province of Canada into separate provinces, Toronto became the capital of Ontario and eventually became Canada's main economic centre.

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.