James Carmichael Smyth FRS FRCP (23 February 1742 – 18 June 1821) was a Scottish physician and medical writer.

Appointed physician to the Middlesex Hospital in 1768, he discovered a method for the prevention of contagion in cases of fever using nitrous acid gas, and wrote several treatises on this subject and on other medical matters.

He was elected a Fellow of the Royal Society in May 1779, and was voted the sum of £5000 by Parliament in 1802 for his work.

The results (published in 1796) of an experiment made at the desire of the Lords Commissioners of the Admiralty, on board the Union hospital ship, to determine the effect of the nitrous acid in destroying contagion, and the safety with which it may be employed were given in a letter addressed to the Right Hon.

[2] His eldest son, James, initially an officer in the Royal Engineers and later Governor of the Bahamas and British Guiana, was created a baronet in 1821.