John Herapath (May 30, 1790 - February 24, 1868) was an English physicist who gave a partial account of the kinetic theory of gases in 1820 though it was neglected by the scientific community at the time.

Herapath's scientific interests started with an attempt to provide a mechanistic explanation for gravity. Motivated by Georges-Louis LeSage's kinetic theory of gravity, he started to consider how a system of colliding particles could give rise to action at a distance. In considering the effect of the high temperatures near the Sun on his gravific particles he was led to a relationship between temperature and particle velocity.

Herapath postulated that the momentum of a particle in a gas is a measure of the absolute temperature of the gas. He used momentum, rather than the kinetic energy on which the later established theory is based, as it seemed to him to avoid some difficulties around whether elastic collisions were possible between indivisible atoms. Apparently ignorant of Daniel Bernoulli's work, he was led to the incorrect, but suggestive, relationship that expresses the product of pressure P and volume V as proportional to the square of his true temperature. The correct relationship is proportional to the absolute temperature, not its square, the error arising from his identification of momentum, rather than energy, with temperature.

He submitted his ideas in a paper to the Royal Society in 1820 where it was peer reviewed by Sir Humphry Davy. Davy had already sympathised with the view that heat was associated with molecular motion rather than with Joseph Black's caloric theory of heat but he rejected Herapath's paper with some coolness, uncomfortable with the implication that there was an absolute zero of temperature at which all motion ceased. Davy may also have had some distaste for the mechanistic Newtonian picture, influenced as he was by the more holistic philosophy of the Romantic movement.

In 1821, Herapath managed to have his paper published in the Annals of Philosophy, a well-read journal that counted Michael Faraday among its regular contributors. However, the paper seems to have attracted little attention other than from James Prescott Joule who presented a short account of the work in 1848, again to little reaction. Meanwhile, Herapath maintained a campaign against Davy and the Royal Society in the correspondence pages of The Times newspaper.

He discovered the Great Comet of 1831 on January 7, 1831.

Herapath became editor of the Railway Magazine which gave him some limited opportunity to publish his scientific ideas. In 1836, he published a calculation of the mean molecular speed in a gas based on his kinetic theory and hence the speed of sound. Joule reproduced his results but is usually incorrectly credited as the originator.

He revised his theories in the 1840s, largely based on the experimental work of Thomas Graham and Henri Victor Regnault, and published it in his two-volume Mathematical Physics (1847).