Mere Christianity is a Christian apologetical book by the British author C. S. Lewis. It was adapted from a series of BBC radio talks made between 1941 and 1944, originally published as three separate volumes: Broadcast Talks (1942), Christian Behaviour (1943), and Beyond Personality (1944). The book consists of four parts: the first presents Lewis's arguments for the existence of God; the second contains his defence of Christian theology, including his notable "Liar, lunatic, or Lord" trilemma; the third has him exploring Christian ethics, among which cardinal and theological virtues; in the final, he writes on the Christian conception of God.

Mere Christianity was published in the United Kingdom by Geoffrey Bles on 7 July 1952. While initial reviews to the book were generally positive, modern reviewers were more critical of it, and its overall reception was relatively mixed. The praise was primarily directed to Lewis's humorous, straightforward style of writing; the criticism was primarily around the validity of his trilemma, which defends the Christian doctrine of the divinity of Jesus, and how he should have considered providing more choices.

Deemed a classic in Lewis's career and religious literature, Mere Christianity has often received a wide readership decades following its release, and contributed to establishing its author's reputation as "one of the most 'original' exponents of the Christian faith" in the 20th century. The work, with Lewis's arguments for God's existence in it, continued to be examined in scholarly circles. Mere Christianity has retained popularity among Christians from various denominations, and appeared in several lists of finest Christian books. Often used as a tool of evangelism, it has been translated into over thirty languages, and cited by a number of public figures as their influence to their conversion to Christianity. Several "biographies" of the book have also been written.