John Gillies Priestley (10 December 1879 – 9 February 1941) was a British physiologist known for his work on gases and human respiration.
He was educated at Eton and Christ Church, Oxford and qualified in medicine at St. Bartholomew's Hospital.
He spent some time working with Wilhelm Falta in Vienna before returning to join as a director of the chemical pathology laboratory at St. Bartholomew's.
Affected by tuberculosis in 1912 he moved to Oxford and served in the Royal Army Medical Corps during World War I.
It was here that he worked with John Scott Haldane on respiration and the concentration of carbon dioxide in the inspired air.