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.