Thornhill is a federal electoral district in Ontario, Canada, that has been represented in the House of Commons of Canada since 1997. It covers its namesake Thornhill neighbourhood, which is split between the Cities of Vaughan and Markham. The Vaughan portion also includes parts of the city east of Highway 400 and south of Rutherford Road, including the largely industrial district of Concord and Vaughan's planned downtown; Vaughan Metropolitan Centre. The part in the City of Markham is restricted its portion of Thornhill itself west of Bayview Avenue. The riding was created in 1996 and the east end of the riding was split off into other ridings in 2012.

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.