Simcoe County is located in the central portion of Southern Ontario, Canada. The county is just north of the Greater Toronto Area, stretching from the shores of Lake Simcoe in the east to Georgian Bay in the west. Simcoe County forms part of the Greater Golden Horseshoe area, a densely populated and industrialized region, centred on the Greater Toronto Area.

The land area of the county is 4,818.93 square kilometres (1,860.60 sq mi).[1] The elevated Niagara Escarpment runs through sections of the western part of the county, and the Minesing Wetlands, a Ramsar Convention wetland of international importance, is located in the central area of the county.

The county administrative centre is on Highway 26 in Midhurst, outside Barrie.

Simcoe County, in particular the former Wendake area near Nottawasaga Bay, was the site of the earliest French exploration and settlement of Ontario; they were the first Europeans in the area. Several historic sites, including Carhagouha and Sainte-Marie among the Hurons, mark the earliest known contacts between the area's traditional Huron population and French missionaries. The Huron capital, Ossossané, was at one time the largest aboriginal settlement in all of North America outside Mexico.