John Joly (/ˈdʒoʊli/; 1 November 1857 – 8 December 1933) was an Irish geologist and physicist known for his development of radiotherapy in the treatment of cancer.

He is also known for developing techniques to more accurately estimate the age of a geological period, based on radioactive elements present in minerals, the uranium–thorium dating.

He worked as a demonstrator in Trinity's Engineering and Physics departments before succeeding William Johnson Sollas in the Chair of Geology and Mineralogy in 1897, a position which he held until his death in 1933 in Dublin.

[4] According to Joly, when the ice is forced to contract by the pressure applied, a film of liquid water is formed, upon which actual slipping occurs.

Along with his friend Henry Horatio Dixon, Joly also put forward the cohesion-tension theory which is now thought to be the main mechanism for the upward movement of water in plants.

The Radium Institute also supplied capillary tubes containing radon to hospitals for some years for use in the treatment of tumours.