William Downie Stewart (1878–1949) was a New Zealand politician of the Reform Party, a Cabinet minister, a lawyer, and Mayor of Dunedin from 1913 to 1914.
He was Minister of Finance in the Liberal-Reform coalition Government of New Zealand from 1931 to 1933
He represented the Dunedin West electorate from 1914 to 1935, when he was defeated by Gervan McMillan. He stood in the 1935 contest as an Independent National candidate.
His father, also William Downie Stewart (1842-1898), had previously represented the Dunedin West electorate.