William Atcheson Stewart (February 26, 1915 – December 8, 1990) was a politician in Ontario, Canada.
He served as a cabinet minister in the governments of Leslie Frost, John Robarts, and Bill Davis.
In 1957, Stewart was elected in a by-election in the riding of Middlesex North to replace Thomas L. Patrick who had died that year.
In 1960, Leslie Frost appointed him to cabinet as a Minister Without Portfolio.
In 1961 he was appointed by new Premier, John Robarts as Minister of Agriculture.