John Hadley (1731 – 5 November 1764) was a British chemist and physician.
John Hadley was appointed in 1756 as the fourth Professor of Chemistry at Cambridge University, the oldest continuously occupied chair of Chemistry in the UK. During his time there he co-operated in 1758 with Benjamin Franklin on a series of experiments to investigate latent heat. They found that a mercury thermometer sprayed with ether which was then evaporated by blowing could fall to −7 degrees celsius in a warm room.
The Professorship was unpaid so Hadley studied medicine and obtained in 1758 a Physick Fellowship. He then moved to London in 1760 and got a post as Assistant Physician at St Thomas Hospital. In 1763 he became full Physician to Charterhouse School and became a Fellow of the Royal College of Physicians.
In 1758 he was elected as a Fellow of the Royal Society. He died in 1764 of a fever at the age of 33.