John Venn, FRS, FSA (4 August 1834 – 4 April 1923) was an English mathematician, logician and philosopher noted for introducing Venn diagrams, which are used in logic, set theory, probability, statistics, and computer science. In 1866, Venn published The Logic of Chance, a groundbreaking book which espoused the frequency theory of probability, arguing that probability should be determined by how often something is forecast to occur as opposed to "educated" assumptions. Venn then further developed George Boole's theories in the 1881 work Symbolic Logic, where he highlighted what would become known as Venn diagrams.

John Venn was born on 4 August 1834 in Kingston upon Hull, Yorkshire, to Martha Sykes and Rev. Henry Venn, who was the rector of the parish of Drypool. His mother died when he was three years old. Venn was descended from a long line of church evangelicals, including his grandfather John Venn. Venn was brought up in a very strict atmosphere at home. His father Henry had played a significant part in the Evangelical movement and he was also the secretary of the Society for Missions to Africa and the East, establishing eight bishoprics overseas. His grandfather was pastor to William Wilberforce of the abolitionist movement, in Clapham.

He began his education in London joining Sir Roger Cholmeley's School, now known as Highgate School, with his brother Henry in September 1846. He moved on to Islington Proprietary School.

In October 1853, he went to Gonville and Caius College, Cambridge. He found the Mathematical Tripos unsuited to his mathematical style, complaining that the handful of private tutors he worked with "always had the Tripos prominently in view". In contrast, Venn wished to investigate interesting ideas beyond the syllabus. Nonetheless, he was Sixth Wrangler upon sitting the exams in January 1857.

Venn experienced, in his words, a "reaction and disgust" to the Tripos which led him to sell his books on mathematics and state that he would never return to the subject. Following his family vocation, he was ordained as an Anglican priest in 1859, serving first at the church in Cheshunt, Hertfordshire, and later in Mortlake, Surrey.

In 1862, he returned to Cambridge as a lecturer in moral science, studying and teaching political economy, philosophy, probability theory and logic. He reacquainted himself with logic and became a leading scholar in the field through his textbooks The Logic of Chance (1866), Symbolic Logic (1881) and The Principles of Empirical or Inductive Logic (1889). His academic writing was influenced by his teaching: he saw Venn diagrams, which he called "Eulerian Circles" and introduced in 1880, as a pedagogical tool. Venn was known for teaching students across multiple Cambridge colleges, which was rare at the time.

In 1883, he resigned from the clergy, having concluded that Anglicanism was incompatible with his philosophical beliefs.

In 1903 he was elected President of the college, a post he held until his death.