Thornhill is a suburban district in the Regional Municipality of York in Ontario, Canada. The western portion of Thornhill is within the City of Vaughan and its eastern portion is within the City of Markham, with Yonge Street forming the boundary between Vaughan and Markham. Thornhill is situated along the northern border of Toronto, centred on Yonge, and is also immediately south of the City of Richmond Hill. Once a police village, Thornhill is still a postal designation. As of 2016, its total population, including both its Vaughan and Markham sections, was 112,719.

Yonge Street is a major arterial route in the Canadian province of Ontario connecting the shores of Lake Ontario in Toronto to Lake Simcoe, a gateway to the Upper Great Lakes. Ontario's first colonial administrator, John Graves Simcoe, named the street for his friend Sir George Yonge, an expert on ancient Roman roads.