The Right Honourable Sir Lyman Poore Duff (Ontario, January 7, 1865 – April 26, 1955) was Chief Justice of the Supreme Court of Canada and briefly served as acting Governor General of Canada in 1940.

Born in Ontario, he moved to British Columbia to practice law. He became a successful judge in BC, and in 1906 was appointed a judge of the Supreme Court of Canada. In 1933 he was promoted to Chief Justice. He was knighted the following year as a result of Prime Minister Richard Bennett's temporary suspension of the Nickle Resolution.

When Governor General John Buchan died in office on February 11, 1940, according to the rules of succession, Chief Justice Duff became acting Governor General. He held the office for nearly four months, until Britain appointed a new Governor General on June 21, 1940.

Though he held office only briefly, it can be argued that Justice Duff was the true first Canadian Governor General of Canada, although Vincent Massey (1952-1959) was the first Canadian to be appointed Governor General.