Thomas McKay[a] (1 September 1792 – 9 October 1855) was a Canadian businessman who was one of the founders of the city of Ottawa, Ontario.

He became partners with John Redpath and their firm did the masonry work on the Lachine Canal near Montreal, they then went on to build the locks on the lower section of the Rideau Canal, between the Rideau River and the Ottawa River at Bytown.

On the western edge of this new land he built in 1838 for himself a limestone Scottish Regency mansion which he named Rideau Hall, and which is today official residence of the Governor General of Canada.

McKay also brought the first railroad to the Ottawa area with the Prescott and Bytown Railway that had its terminus at a station near Sussex Drive just south of New Edinburgh.

McKay entered politics serving on Bytown's city council, and then the Legislative Assembly of Upper Canada from 1834 to 1841.