Hon. William Duncan Herridge, P.C., (September 18 1888 - September 21 1961) was a Canadian politician and diplomat. Herridge was Canada's envoy to the United States with the title Envoy Extraordinary and Minister Plenipotentiary for Canada in the United States of America, from 1931 to 1935. He was appointed to the position by his brother-in-law, Conservative Prime Minister R.B. Bennett, after having acted as speechwriter and policy advisor to Bennett during the 1930 Canadian election that brought the Tories to power.

While stationed in Washington DC, Herridge was impressed by Franklin Delano Roosevelt and his New Deal, and convinced Bennett to attempt to adopt similar policies in Canada for combatting the Great Depression. Bennett's turn towards reform occurred too late in his term, however, and his government was soundly defeated in the 1935 Canadian election.

Herridge returned to Canada and, in 1939, launched the New Democracy party, which advocated monetary reform and government intervention in the economy. The party's positions were similar to those of the Social Credit Party of Canada. The two parties ran a joint slate in the 1940 Canadian election under the New Democracy banner with Herridge as the lead candidate. Herridge, however, came in third in his riding of Kindersley, Saskatchewan with 30% of the vote and thus failed to win a seat in the Canadian House of Commons. The only New Democracy MPs elected were those who had previously been elected to parliament for the Social Credit party. After the election the New Democracy party folded and its MPs reverted to their previous identity as Social Credit members.