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.

The Regional Municipality of York, also called York Region, is a regional municipality in Southern Ontario, Canada, between Lake Simcoe and Toronto. The region was established after the passing of then Bill 102, An Act to Establish The Regional Municipality of York, in 1970. It replaced the former York County in 1971, and is part of the Greater Toronto Area and the inner ring of the Golden Horseshoe. The regional government is headquartered in Newmarket.