Sir Frank Gavan Duffy KCMG PC KC (29 February 1852 – 29 July 1936) was an Australian judge who served as the fourth Chief Justice of Australia, in office from 1931 to 1935.

In 1869, Duffy returned to Australia and attended the University of Melbourne, graduating in 1872 with a Bachelor of Arts.

In 1908, with William Ah Ket, he successfully represented James Minahan before the High Court, in a landmark decision which recognised the principle of legality for the first time in Australian common law.

As Chief Justice on the infrequent occasions he penned his own judgments they were extremely brief and he sat on less than half of the full court cases.

[6] He had planned to deliver a series of lectures on Australian constitutional law at Harvard University's tercentenary early in 1936, but was too ill.

[7] He was survived by his wife and three sons,[1] among them Charles Leonard Gavan Duffy, later a Justice of the Supreme Court of Victoria.