King's Highway 400, commonly referred to as Highway 400, historically as the Toronto–Barrie Highway, and colloquially as the 400, is a 400-series highway in the Canadian province of Ontario linking the city of Toronto in the urban and agricultural south of the province with the scenic and sparsely populated central and northern regions. The portion of the highway between Toronto and Lake Simcoe roughly traces the route of the Toronto Carrying-Place Trail, a historic trail between the Lower and Upper Great Lakes. North of Highway 12, in combination with Highway 69, it forms a branch of the Trans-Canada Highway (TCH), the Georgian Bay Route, and is part of the highest-capacity route from southern Ontario to the Canadian West, via a connection with the mainline of the TCH in Sudbury. The highway also serves as the primary route from Toronto to southern Georgian Bay and Muskoka, areas collectively known as cottage country. The highway is patrolled by the Ontario Provincial Police and has a speed limit of 100 km/h (62 mph), except for the section south of Highway 401, where the speed limit is 80 km/h (50 mph).

The Trans-Canada Highway is a transcontinental federal–provincial highway system that travels through all ten provinces of Canada, from the Pacific Ocean on the west coast to the Atlantic Ocean on the east coast. The main route spans 7,476 km (4,645 mi) across the country, one of the longest routes of its type in the world. The highway system is recognizable by its distinctive white-on-green maple leaf route markers, although there are small variations in the markers in some provinces.