List of National Historic Sites of Canada in Ontario

This is a list of National Historic Sites () in the province of Ontario. There are 273 sites designated in Ontario, 39 of which are administered by Parks Canada (identified below and on the cluster pages listed below by the beaver icon . Of all provinces and territories, Ontario has the greatest number of National Historic Sites, and the largest number under Parks Canada administration, with a dense concentration in southern Ontario. The five largest clusters are listed separately:
Numerous National Historic Events also occurred across Ontario, and are identified at places associated with them, using the same style of federal plaque which marks National Historic Sites. Several National Historic Persons are commemorated throughout the province in the same way. The markers do not indicate which designation—a Site, Event, or Person—a subject has been given. The Rideau Canal is a Site, for example, while the Welland Canal is an Event. The cairn and plaque to John Macdonell does not refer to a National Historic Person, but is erected because his home, Glengarry House, is a National Historic Site. Similarly, the plaque to John Guy officially marks not a Person, but an Event—the Landing of John Guy.
This list uses names designated by the national "Historic Sites and Monuments Board", which may differ from other names for these sites.