Markham—Thornhill is a federal electoral district in Ontario, Canada, that has been represented in the House of Commons of Canada since 2015. It encompasses a portion of Ontario previously included in the electoral districts of Markham—Unionville and Thornhill. Markham—Thornhill was created by 2012 federal electoral boundaries redistribution and was legally defined in the 2013 representation order. It came into effect upon the dropping of the writs for the 2015 federal election.