John Beresford Leathes DSc, MA, FRS,FRCS, FRCP (5 November 1864 – 14 September 1956) was a British physiologist and an early biochemist. He was the son of Hebrew scholar Stanley Leathes, and the brother of the poet, historian and First Civil Service Commissioner Sir Stanley Mordaunt Leathes.
The son of the Rev. Stanley Leathes, the Professor of Hebrew at King's College London and his wife Matilda (née Butt), a descendant of a Dr. Butt who was a physician to Henry VIII, his older brother was the poet, historian and senior Civil Servant Sir Stanley Mordaunt Leathes. John Beresford Leathes was educated in the Classics at Winchester College from 1878 to 1883. The College at that time possessed no science facilities, so he received little science teaching there. When Gladstone visited the College Leathes, welcoming him formally Ad Portas as Prefect of Hall, delivered a speech to him in Latin, to which Gladstone responded in English.
In 1884 he went up to New College, Oxford where he obtained a second-class degree in Classics. Rejecting his father's wishes to become ordained in the Church of England, Leathes instead studied Medicine at Guy's Hospital, walking 12 miles there and back each day from his aunt's home in Highgate. He qualified BMBCh (Oxford) in 1893, and in 1894 passed FRCS.